feat: admin image generation via Cloudflare Workers AI

- Add cfai/image.go: ImageGenClient with GenerateImage, GenerateImageFromReference, AllImageModels (9 models)
- Add handlers_image.go: GET /api/admin/image-gen/models + POST /api/admin/image-gen (JSON + multipart)
- Wire ImageGen client in main.go + server.go Dependencies
- Add admin/image-gen SvelteKit page: type toggle, model selector, prompt, reference img2img, advanced options, result panel, history, save-as-cover, download
- Add SvelteKit proxy route api/admin/image-gen forwarding to Go backend
- Add admin_nav_image_gen message key to all 5 locale files
- Add Image Gen nav link to admin layout

backend/internal/backend/handlers_image.go

@@ -0,0 +1,234 @@
+package backend
+
+import (
+	"encoding/base64"
+	"encoding/json"
+	"fmt"
+	"io"
+	"net/http"
+	"strings"
+
+	"github.com/libnovel/backend/internal/cfai"
+)
+
+// handleAdminImageGenModels handles GET /api/admin/image-gen/models.
+// Returns the list of supported Cloudflare AI image generation models.
+func (s *Server) handleAdminImageGenModels(w http.ResponseWriter, r *http.Request) {
+	if s.deps.ImageGen == nil {
+		jsonError(w, http.StatusServiceUnavailable, "image generation not configured (CFAI_ACCOUNT_ID/CFAI_API_TOKEN missing)")
+		return
+	}
+	models := s.deps.ImageGen.Models()
+	writeJSON(w, 0, map[string]any{"models": models})
+}
+
+// imageGenRequest is the JSON body for POST /api/admin/image-gen.
+type imageGenRequest struct {
+	// Prompt is the text description of the desired image.
+	Prompt string `json:"prompt"`
+
+	// Model is the CF Workers AI model ID (e.g. "@cf/black-forest-labs/flux-2-dev").
+	// Defaults to the recommended model for the given type.
+	Model string `json:"model"`
+
+	// Type is either "cover" or "chapter".
+	Type string `json:"type"`
+
+	// Slug is the book slug. Required for cover; required for chapter.
+	Slug string `json:"slug"`
+
+	// Chapter number (1-based). Required when type == "chapter".
+	Chapter int `json:"chapter"`
+
+	// ReferenceImageB64 is an optional base64-encoded PNG/JPEG reference image.
+	// When present the img2img path is used.
+	ReferenceImageB64 string `json:"reference_image_b64"`
+
+	// NumSteps overrides inference steps (default 20).
+	NumSteps int `json:"num_steps"`
+
+	// Width / Height override output dimensions (0 = model default).
+	Width  int `json:"width"`
+	Height int `json:"height"`
+
+	// Guidance overrides prompt guidance scale (0 = model default).
+	Guidance float64 `json:"guidance"`
+
+	// Strength for img2img: 0.0–1.0, default 0.75.
+	Strength float64 `json:"strength"`
+
+	// SaveToCover when true stores the result as the book cover in MinIO
+	// (overwriting any existing cover) and sets the book's cover URL.
+	// Only valid when type == "cover".
+	SaveToCover bool `json:"save_to_cover"`
+}
+
+// imageGenResponse is the JSON body returned by POST /api/admin/image-gen.
+type imageGenResponse struct {
+	// ImageB64 is the generated image as a base64-encoded PNG string.
+	ImageB64 string `json:"image_b64"`
+	// ContentType is "image/png" or "image/jpeg".
+	ContentType string `json:"content_type"`
+	// Saved indicates whether the image was persisted to MinIO.
+	Saved bool `json:"saved"`
+	// CoverURL is the URL the cover is now served from (only set when Saved==true).
+	CoverURL string `json:"cover_url,omitempty"`
+	// Model is the model that was used.
+	Model string `json:"model"`
+	// Bytes is the raw image size in bytes.
+	Bytes int `json:"bytes"`
+}
+
+// handleAdminImageGen handles POST /api/admin/image-gen.
+//
+// Generates an image using Cloudflare Workers AI and optionally stores it.
+// Multipart/form-data is also accepted so the reference image can be uploaded
+// directly; otherwise the reference is expected as base64 JSON.
+func (s *Server) handleAdminImageGen(w http.ResponseWriter, r *http.Request) {
+	if s.deps.ImageGen == nil {
+		jsonError(w, http.StatusServiceUnavailable, "image generation not configured (CFAI_ACCOUNT_ID/CFAI_API_TOKEN missing)")
+		return
+	}
+
+	var req imageGenRequest
+	var refImageData []byte
+
+	ct := r.Header.Get("Content-Type")
+	if strings.HasPrefix(ct, "multipart/form-data") {
+		// Multipart: parse JSON fields from a "json" part + optional "reference" file part.
+		if err := r.ParseMultipartForm(32 << 20); err != nil {
+			jsonError(w, http.StatusBadRequest, "parse multipart: "+err.Error())
+			return
+		}
+		if jsonPart := r.FormValue("json"); jsonPart != "" {
+			if err := json.Unmarshal([]byte(jsonPart), &req); err != nil {
+				jsonError(w, http.StatusBadRequest, "parse json field: "+err.Error())
+				return
+			}
+		}
+		if f, _, err := r.FormFile("reference"); err == nil {
+			defer f.Close()
+			refImageData, _ = io.ReadAll(f)
+		}
+	} else {
+		if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
+			jsonError(w, http.StatusBadRequest, "parse body: "+err.Error())
+			return
+		}
+		if req.ReferenceImageB64 != "" {
+			var decErr error
+			refImageData, decErr = base64.StdEncoding.DecodeString(req.ReferenceImageB64)
+			if decErr != nil {
+				// Try std without padding
+				refImageData, decErr = base64.RawStdEncoding.DecodeString(req.ReferenceImageB64)
+				if decErr != nil {
+					jsonError(w, http.StatusBadRequest, "decode reference_image_b64: "+decErr.Error())
+					return
+				}
+			}
+		}
+	}
+
+	if strings.TrimSpace(req.Prompt) == "" {
+		jsonError(w, http.StatusBadRequest, "prompt is required")
+		return
+	}
+	if req.Type != "cover" && req.Type != "chapter" {
+		jsonError(w, http.StatusBadRequest, `type must be "cover" or "chapter"`)
+		return
+	}
+	if req.Slug == "" {
+		jsonError(w, http.StatusBadRequest, "slug is required")
+		return
+	}
+	if req.Type == "chapter" && req.Chapter <= 0 {
+		jsonError(w, http.StatusBadRequest, "chapter must be > 0 when type is chapter")
+		return
+	}
+
+	// Resolve model
+	model := cfai.ImageModel(req.Model)
+	if model == "" {
+		if req.Type == "cover" {
+			model = cfai.DefaultImageModel
+		} else {
+			model = cfai.ImageModelFlux2Klein4B
+		}
+	}
+
+	imgReq := cfai.ImageRequest{
+		Prompt:   req.Prompt,
+		Model:    model,
+		NumSteps: req.NumSteps,
+		Width:    req.Width,
+		Height:   req.Height,
+		Guidance: req.Guidance,
+		Strength: req.Strength,
+	}
+
+	s.deps.Log.Info("admin: image gen requested",
+		"type", req.Type, "slug", req.Slug, "chapter", req.Chapter,
+		"model", model, "has_reference", len(refImageData) > 0)
+
+	var imgData []byte
+	var genErr error
+	if len(refImageData) > 0 {
+		imgData, genErr = s.deps.ImageGen.GenerateImageFromReference(r.Context(), imgReq, refImageData)
+	} else {
+		imgData, genErr = s.deps.ImageGen.GenerateImage(r.Context(), imgReq)
+	}
+	if genErr != nil {
+		s.deps.Log.Error("admin: image gen failed", "err", genErr)
+		jsonError(w, http.StatusBadGateway, "image generation failed: "+genErr.Error())
+		return
+	}
+
+	contentType := sniffImageContentType(imgData)
+
+	// ── Optional persistence ──────────────────────────────────────────────────
+	var saved bool
+	var coverURL string
+
+	if req.SaveToCover && req.Type == "cover" && s.deps.CoverStore != nil {
+		if err := s.deps.CoverStore.PutCover(r.Context(), req.Slug, imgData, contentType); err != nil {
+			s.deps.Log.Error("admin: save generated cover failed", "slug", req.Slug, "err", err)
+			// Non-fatal: still return the image
+		} else {
+			saved = true
+			coverURL = fmt.Sprintf("/api/cover/local/%s", req.Slug)
+			s.deps.Log.Info("admin: generated cover saved", "slug", req.Slug, "bytes", len(imgData))
+		}
+	}
+
+	// Encode result as base64
+	b64 := base64.StdEncoding.EncodeToString(imgData)
+
+	writeJSON(w, 0, imageGenResponse{
+		ImageB64:    b64,
+		ContentType: contentType,
+		Saved:       saved,
+		CoverURL:    coverURL,
+		Model:       string(model),
+		Bytes:       len(imgData),
+	})
+}
+
+// sniffImageContentType returns the MIME type of the image bytes.
+func sniffImageContentType(data []byte) string {
+	if len(data) >= 4 {
+		// PNG: 0x89 P N G
+		if data[0] == 0x89 && data[1] == 0x50 && data[2] == 0x4e && data[3] == 0x47 {
+			return "image/png"
+		}
+		// JPEG: FF D8 FF
+		if data[0] == 0xFF && data[1] == 0xD8 && data[2] == 0xFF {
+			return "image/jpeg"
+		}
+		// WebP: RIFF....WEBP
+		if len(data) >= 12 && data[0] == 'R' && data[1] == 'I' && data[2] == 'F' && data[3] == 'F' &&
+			data[8] == 'W' && data[9] == 'E' && data[10] == 'B' && data[11] == 'P' {
+			return "image/webp"
+		}
+	}
+	return "image/png"
+}

backend/internal/backend/server.go

@@ -73,6 +73,9 @@ type Dependencies struct {
 	// CFAI is the Cloudflare Workers AI TTS client (used for voice sample
 	// generation and audio-stream live TTS; audio task generation is done by the runner).
 	CFAI cfai.Client
+	// ImageGen is the Cloudflare Workers AI image generation client.
+	// If nil, image generation endpoints return 503.
+	ImageGen cfai.ImageGenClient
 	// Log is the structured logger.
 	Log *slog.Logger
 }
@@ -183,6 +186,10 @@ func (s *Server) ListenAndServe(ctx context.Context) error {
 	mux.HandleFunc("POST /api/admin/audio/bulk", s.handleAdminAudioBulk)
 	mux.HandleFunc("POST /api/admin/audio/cancel-bulk", s.handleAdminAudioCancelBulk)
 
+	// Admin image generation endpoints
+	mux.HandleFunc("GET /api/admin/image-gen/models", s.handleAdminImageGenModels)
+	mux.HandleFunc("POST /api/admin/image-gen", s.handleAdminImageGen)
+
 	// Voices list
 	mux.HandleFunc("GET /api/voices", s.handleVoices)