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/sync/annotations.go

@@ -590,6 +590,9 @@ type SaveBookmarkRequest struct {
 	Source           string
 	ModifiedAt       time.Time
 	DeviceSyncData   json.RawMessage
+	// OriginSource records the client that created the bookmark (set once
+	// at insert; unlike Source it is not updated by later writers).
+	OriginSource string
 	// DedupKey overrides the computed key for device echoes (see
 	// SaveHighlightRequest).
 	DedupKey string
@@ -624,9 +627,9 @@ func (s *AnnotationService) SaveBookmark(ctx context.Context, req SaveBookmarkRe
 		if !incomingNewerThanTombstone(req.ModifiedAt, existing.DeletedAt, existing.LastModifiedAt) {
 			return &SaveBookmarkResult{Bookmark: existing, Outcome: SaveOutcomeDeleted}, nil
 		}
-		// Newer than the tombstone: a deliberate re-create. Resurrect via the
-		// LWW update instead of INSERT (the tombstoned row still holds the
-		// UNIQUE(media_item_id, user_id, title) slot).
+		// Newer than the tombstone: a deliberate re-create at the same
+		// location. Resurrect via the LWW update so the row keeps its id
+		// and origin.
 		return s.applyBookmarkLWW(ctx, req, existing, dedupKey)
 	}
 
@@ -656,6 +659,7 @@ func (s *AnnotationService) createBookmark(ctx context.Context, req SaveBookmark
 		LastModifiedAt:     pgtype.Timestamptz{Time: modifiedAt, Valid: true},
 		LastModifiedSource: pgtype.Text{String: req.Source, Valid: req.Source != ""},
 		DeviceSyncData:     deviceData,
+		OriginSource:       pgText(req.OriginSource),
 	})
 	if err != nil {
 		return nil, fmt.Errorf("create bookmark: %w", err)