feat(bookmarks): location identity, origin provenance, KOReader-style labels

- Drop UNIQUE(media_item_id,user_id,title): titles are display labels
  shared verbatim across clients; same-title bookmarks on different pages
  now coexist instead of 500ing (deleting over a tombstone no longer
  blocks future creates with that title)
- Add origin_source column recording the creating client (android/web/
  koreader), set once at insert, exposed in API responses
- Web reader auto-title mirrors KOReader's 'in <chapter>' convention,
  falling back to 'Bookmark'; adds bookmark rename in the drawer

internal/database/queries/queries.sql

@@ -898,9 +898,9 @@ INSERT INTO media_bookmarks (
     cfi_position, title, position, notes,
     percentage_location, epubcfi_location, chapter_reference,
     dedup_key, last_modified_at, last_modified_source,
-    device_sync_data
+    device_sync_data, origin_source
 ) VALUES (
-    $1, $2, $3, $4, $5, $6, $7, $8, $9, $10, $11, $12, $13, $14, $15
+    $1, $2, $3, $4, $5, $6, $7, $8, $9, $10, $11, $12, $13, $14, $15, $16
 ) RETURNING *;
 
 -- name: UpdateMediaBookmarkForSync :one