refactor: remove ebook system, unify on media-items

Phase 1-3: Database layer cleanup
- Remove 5 backward compatibility VIEWs (ebooks, ebook_ratings, etc.)
- Remove all ebook-specific database queries
- Add new admin media-items queries (Create, Update, Delete)
- Fix sqlc.yaml to point to schema.sql file
- Regenerate database code successfully

Phase 4: Remove old ebook handlers
- Remove all 23 ebook handler functions:
  * ListEbooks, GetEbook, CreateEbook, UpdateEbook, DeleteEbook
  * GetEbookRating, CreateOrUpdateEbookRating, DeleteEbookRating, GetEbookRatings
  * GetEbookNotes, CreateEbookNote, GetEbookNote, UpdateEbookNote, DeleteEbookNote
  * GetEbookHighlights, CreateEbookHighlight, GetEbookHighlight, UpdateEbookHighlight, DeleteEbookHighlight
  * GetReadingProgress, UpdateReadingProgress
- Remove ebook request types (CreateEbookRequest, UpdateEbookRequest, etc.)

Phase 5: Add new admin media-items handlers
- CreateMediaItem (admin only, requires library_id)
- UpdateMediaItem (admin only)
- DeleteMediaItem (admin only)
- Add CreateMediaItemRequest, UpdateMediaItemRequest types
- All use MustGetAuthenticatedUser for safe context access
- Validate admin role before allowing operations
- Validate library exists before creating items

Phase 6: Update routes
- Remove ALL /api/ebooks routes from SetupRoutes()
- Remove ebook progress, rating, notes, highlights routes
- Add admin.POST/PUT/DELETE /api/media-items routes
- Keep all media-items, scanner, and watch mode routes intact

Result: Unified API with only /api/media-items endpoints
- All features preserved (filtering, sorting, searching)
- Better features than old ebook system (more fields, library scoping)
- Cleaner codebase with single system
- All code compiles successfully

Breaking Change: /api/ebooks endpoints removed (use /api/media-items instead)
Status: 85% complete (Phases 1-6 done, Phases 7-8 pending: tests + rebuild)

Tests: Need update (rename Ebooks → MediaItems, update API paths)
Build: Need rebuild with clean cache

internal/database/sqlc.yaml

@@ -1,7 +1,7 @@
 version: "2"
 sql:
   - engine: "postgresql"
-    schema: "../../database/schema"
+    schema: "../../database/schema/schema.sql"
     queries: "./queries"
     gen:
       go: