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/handlers/media.go

@@ -152,6 +152,9 @@ type CreateMediaBookmarkRequest struct {
 	ChapterNumber    int32   `json:"chapter_number"`
 	Percentage       float64 `json:"percentage"`
 	ChapterReference int32   `json:"chapter_reference"`
+	// Origin labels the creating client for display ("android", "web").
+	// Optional: defaults to "web" for browser callers.
+	Origin string `json:"origin" validate:"omitempty,max=30"`
 }
 
 // UpdateMediaBookmarkRequest represents the request for updating a media bookmark
@@ -1750,6 +1753,10 @@ func (mh *MediaHandler) CreateMediaBookmark(c *echo.Context) error {
 	// The sync-aware path (dedup + LWW + tombstones) is preferred; fall back
 	// to the plain query when the service isn't wired (e.g. some tests).
 	if mh.annotationSvc != nil {
+		origin := req.Origin
+		if origin == "" {
+			origin = "web"
+		}
 		result, err := mh.annotationSvc.SaveBookmark(c.Request().Context(), wsync.SaveBookmarkRequest{
 			MediaItemID:      pgtype.UUID{Bytes: mediaUUID, Valid: true},
 			UserID:           pgtype.UUID{Bytes: userUUID, Valid: true},
@@ -1762,6 +1769,7 @@ func (mh *MediaHandler) CreateMediaBookmark(c *echo.Context) error {
 			PercentageLoc:    req.Percentage,
 			ChapterReference: req.ChapterReference,
 			Source:           "web",
+			OriginSource:     origin,
 			ModifiedAt:       time.Now(),
 		})
 		if err != nil {