feat: option A — visibility gating + author submission system

Content visibility:
- Add `visibility` field to books ("public" | "admin_only"); new migration
  backfills all existing scraped books to admin_only
- Meilisearch: add visibility as filterable attribute; catalogue/search
  endpoints filter to public-only for non-admin requests
- Admin users identified by bearer token bypass the filter and see all books
- All PocketBase discovery queries (trending, recommended, recently-updated,
  audio shelf, discover, subscription feed) now filter to visibility=public
- New scraped books default to admin_only; WriteMetadata preserves existing
  visibility on PATCH (never overwrites)

Author submission:
- POST /api/admin/books/submit — creates a public book with submitted_by
- PATCH /api/admin/books/{slug}/publish / unpublish — toggle visibility
- SvelteKit proxies: /api/admin/books/[slug]/publish|unpublish
- /api/books/[slug] endpoint for admin book lookup

Frontend:
- backendFetchAdmin() helper sends admin token on any path
- Catalogue server load uses admin fetch when user is admin
- /submit page: author submission form with genre picker and rights assertion
- "Publish" nav link shown to all logged-in users
- Admin catalogue-tools: visibility management panel (load book by slug, toggle)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/internal/domain/domain.go

@@ -7,6 +7,12 @@ import "time"
 
 // ── Book types ────────────────────────────────────────────────────────────────
 
+// Visibility values for BookMeta.Visibility.
+const (
+	VisibilityPublic    = "public"     // visible to all users
+	VisibilityAdminOnly = "admin_only" // visible only to admin users (e.g. scraped content)
+)
+
 // BookMeta carries all bibliographic information about a novel.
 type BookMeta struct {
 	Slug          string   `json:"slug"`
@@ -27,6 +33,12 @@ type BookMeta struct {
 	// Archived is true when the book has been soft-deleted by an admin.
 	// Archived books are excluded from all public search and catalogue responses.
 	Archived bool `json:"archived,omitempty"`
+	// Visibility controls who can see this book.
+	// "public" = all users; "admin_only" = admin only (default for scraped content).
+	Visibility string `json:"visibility,omitempty"`
+	// SubmittedBy is the app_users ID of the author who submitted this book,
+	// or empty for scraped books.
+	SubmittedBy string `json:"submitted_by,omitempty"`
 }
 
 // CatalogueEntry is a lightweight book reference returned by catalogue pages.