feat(v3): add v3 stack — backend rewrite, renamed env vars, docs

- New Go backend binary (backend + runner) replacing old scraper/
- Rename SCRAPER_API_URL → BACKEND_API_URL in UI env and docker-compose
- Rename scraperFetch → backendFetch across all 19 UI server files
- Remove SCRAPER_PROXY env var and proxy transport from browser.Config
- Add Meilisearch, Valkey, Caddy to docker-compose
- Add docs/: api-endpoints.md, request-flow.mermaid.md, data-flow.mermaid.md

v3/backend/internal/scraper/scraper.go

@@ -0,0 +1,58 @@
+// Package scraper defines the NovelScraper interface and its sub-interfaces.
+// Domain types live in internal/domain — this package only defines the scraping
+// contract so that novelfire and any future scrapers can be swapped freely.
+package scraper
+
+import (
+	"context"
+
+	"github.com/libnovel/backend/internal/domain"
+)
+
+// CatalogueProvider can enumerate every novel available on a source site.
+type CatalogueProvider interface {
+	ScrapeCatalogue(ctx context.Context) (<-chan domain.CatalogueEntry, <-chan error)
+}
+
+// MetadataProvider can extract structured book metadata from a novel's landing page.
+type MetadataProvider interface {
+	ScrapeMetadata(ctx context.Context, bookURL string) (domain.BookMeta, error)
+}
+
+// ChapterListProvider can enumerate all chapters of a book.
+type ChapterListProvider interface {
+	ScrapeChapterList(ctx context.Context, bookURL string) ([]domain.ChapterRef, error)
+}
+
+// ChapterTextProvider can extract the readable text from a single chapter page.
+type ChapterTextProvider interface {
+	ScrapeChapterText(ctx context.Context, ref domain.ChapterRef) (domain.Chapter, error)
+}
+
+// RankingProvider can enumerate novels from a ranking page.
+type RankingProvider interface {
+	// ScrapeRanking pages through up to maxPages ranking pages.
+	// maxPages <= 0 means all pages.
+	ScrapeRanking(ctx context.Context, maxPages int) (<-chan domain.BookMeta, <-chan error)
+}
+
+// NovelScraper is the full interface a concrete novel source must implement.
+type NovelScraper interface {
+	CatalogueProvider
+	MetadataProvider
+	ChapterListProvider
+	ChapterTextProvider
+	RankingProvider
+
+	// SourceName returns the human-readable name of this scraper, e.g. "novelfire.net".
+	SourceName() string
+}
+
+// Selector describes how to locate an element in an HTML document.
+type Selector struct {
+	Tag      string
+	Class    string
+	ID       string
+	Attr     string
+	Multiple bool
+}