docs: update documentation for media scanner naming

- Update development.md with new file names (scanner.go, media_scanner.go)
- Update API reference: "Book/ebook operations" -> "Media item operations"
- Remove historical migration comments from schema:
  - Simplify media_items table comment
  - Remove backward compatibility comments for reading_progress and media_ratings
  - Remove note about backward compatibility views
- Remove historical comment from queries.sql about ebook folders

docs/contributing/development.md

@@ -32,7 +32,7 @@ bookhoard/
 **Handlers** (`internal/handlers/`):
 - `auth.go` - Authentication & user management
 - `library.go` - Library CRUD operations
-- `ebook.go` - Media item operations
+- `scanner.go` - Media scanning operations
 - `media.go` - Media downloads, shelves
 - `koreader.go` - KOReader sync protocol
 - `kobo.go` - Kobo sync protocol
@@ -63,7 +63,7 @@ bookhoard/
 
 **Services** (`internal/services/`):
 - `library_service.go` - Library operations
-- `ebook_scanner.go` - File scanning & metadata extraction
+- `media_scanner.go` - File scanning & metadata extraction
 - `worker.go` - Job queue worker pool
 - `scheduler.go` - Scheduled task manager
 - `collection_service.go` - Collection rules processing