feat(api): add POST /api/media-items/:id/rescan endpoint

Admin-only endpoint that re-extracts metadata and cover art for a single
book from the file on disk via MediaScanner.RescanMediaItem and returns
the updated media item. Normal library scans skip unchanged files, so
this gives a targeted way to backfill covers for previously imported
books. Includes a Bruno request alongside the existing media-items
collection.

internal/handlers/media.go

@@ -1309,6 +1309,42 @@ func (mh *MediaHandler) UpdateMediaItem(c *echo.Context) error {
 	return c.JSON(http.StatusOK, item)
 }
 
+// RescanMediaItem handles POST /api/media-items/:id/rescan (admin only)
+func (mh *MediaHandler) RescanMediaItem(c *echo.Context) error {
+	user := MustGetAuthenticatedUser(c)
+
+	if user.Role != "admin" {
+		return c.JSON(http.StatusForbidden, map[string]string{"error": "admin access required"})
+	}
+
+	mediaID := c.Param("id")
+	mediaUUID, err := uuid.Parse(mediaID)
+	if err != nil {
+		return c.JSON(http.StatusBadRequest, map[string]string{"error": "invalid media item id"})
+	}
+
+	if _, err := mh.db.GetMediaItem(c.Request().Context(), pgtype.UUID{Bytes: mediaUUID, Valid: true}); err != nil {
+		if errors.Is(err, pgx.ErrNoRows) {
+			return c.JSON(http.StatusNotFound, map[string]string{"error": "media item not found"})
+		}
+		return c.JSON(http.StatusInternalServerError, map[string]string{"error": err.Error()})
+	}
+
+	scanner := services.NewMediaScanner(mh.db)
+	defer scanner.Close()
+
+	if err := scanner.RescanMediaItem(c.Request().Context(), pgtype.UUID{Bytes: mediaUUID, Valid: true}); err != nil {
+		return c.JSON(http.StatusInternalServerError, map[string]string{"error": err.Error()})
+	}
+
+	item, err := mh.db.GetMediaItem(c.Request().Context(), pgtype.UUID{Bytes: mediaUUID, Valid: true})
+	if err != nil {
+		return c.JSON(http.StatusInternalServerError, map[string]string{"error": err.Error()})
+	}
+
+	return c.JSON(http.StatusOK, item)
+}
+
 // DeleteMediaItem handles DELETE /api/media-items/:id (admin only)
 func (mh *MediaHandler) DeleteMediaItem(c *echo.Context) error {
 	user := MustGetAuthenticatedUser(c)

internal/router/media.go

@@ -60,6 +60,7 @@ func registerMediaRoutes(cfg *Config) {
 	// Admin-only media routes
 	admin.POST("/media-items", cfg.MediaHandler.CreateMediaItem)
 	admin.PUT("/media-items/:id", cfg.MediaHandler.UpdateMediaItem)
+	admin.POST("/media-items/:id/rescan", cfg.MediaHandler.RescanMediaItem)
 	admin.DELETE("/media-items/:id", cfg.MediaHandler.DeleteMediaItem)
 
 	// Shelf management (protected)