fix(media): gate file serving by library visibility; proper download URL

ServeFile previously authenticated only ("any logged-in user") and never
checked that the user can actually see the library owning the file, so
knowing a library UUID + path was enough to fetch content from hidden
libraries. Library visibility is the permission model - the library is
what grants access to its media.

- ServeFile now resolves two URL forms through one flow:
  /uploads/library-{id}/{path}   (covers, reader files)
  /api/media-items/{id}/download (explicit book download, new)
  The item form looks up the media item, derives its library and file
  path, and adds a Content-Disposition attachment header.
- Both forms enforce GetUserVisibleLibraries for the authenticated
  user, mirroring the OPDS download handler (403 when not visible).
- Deleted the dead MediaHandler.DownloadBook handler (never routed).

Also widen media_highlights.start_position/end_position from
VARCHAR(100) to TEXT: the API handlers validate up to 1000 characters
(full Readium locators, KOReader CRE xpointers) but the column rejected
anything longer at the database layer. Metadata-only change applied
idempotently at startup; existing rows are untouched.

Verified against the running server: download 200 + attachment headers
+ epub bytes, unauthenticated 401, user hidden from the library 403 on
both URL forms, visible user 200, covers unchanged, and a 334-char
locator JSON now round-trips through the highlights API.

internal/router/media.go

@@ -15,6 +15,9 @@ func registerMediaRoutes(cfg *Config) {
 	// Media item routes (all authenticated users)
 	protected.GET("/media-items", cfg.MediaHandler.ListMediaItems)
 	protected.GET("/media-items/:id", cfg.MediaHandler.GetMediaItem)
+	// Book download endpoint - same ServeFile flow as /uploads/library-:id/*
+	// (JWT + library-visibility gated), addressed by media item ID.
+	protected.GET("/media-items/:id/download", cfg.MediaHandler.ServeFile)
 
 	// Media rating routes (all authenticated users)
 	protected.POST("/media-items/:id/rating", cfg.MediaHandler.CreateMediaRating)