fix(scan): only resurrect discards when file changed; add Saved toast

The scan_folder resurrect path was unflagging every discarded photo on
every backend boot. start_initial_scan fires scan_all_source_roots on
container start, which fans out scan_folder for every source root,
which walked every file and silently set is_discarded=False on rows
whose file was still on disk -- so every deploy wiped the user's
discard decisions. Today's series of resurrect log lines for
admin/Photos came from that path, not from any actual user re-upload.

Gate the resurrect on os.path.getmtime(file) > discarded_at so the
WebDAV-DELETE-then-re-upload and trashbin-restore-via-PUT-overwrite
flows still trigger (those rewrite the file and bump mtime), but
routine sweeps respect the user's intent. Rows with discarded_at NULL
(legacy) fall through to skipped -- preserve intent over cleanup.

While there: add a Saved toast to the single-photo updateMutation.
The previous patch made cache writes synchronous, which removed the
visible save delay but also removed any signal that the change was
actually persisted. Toast picks a per-field label from the patched
keys (Title updated / Date updated / etc.) and falls back to a count
for multi-field saves.

frontend/src/components/sidebar/PhotoInfoPanel.tsx

@@ -181,7 +181,10 @@ export function PhotoInfoPanel({ photoId, darkTheme = false }: PhotoInfoPanelPro
       color_label?: string | null
       taken_at?: string
     }) => photosApi.update(photoId, data),
-    onSuccess: (updated: Partial<PhotoDetails> & { id: string }) => {
+    onSuccess: (
+      updated: Partial<PhotoDetails> & { id: string },
+      variables,
+    ) => {
       queryClient.setQueryData<PhotoDetails>(['photo', photoId], (prev) =>
         prev ? { ...prev, ...updated } : (updated as PhotoDetails),
       )
@@ -194,6 +197,35 @@ export function PhotoInfoPanel({ photoId, darkTheme = false }: PhotoInfoPanelPro
       )
       queryClient.invalidateQueries({ queryKey: ['photos'] })
       queryClient.invalidateQueries({ queryKey: LIBRARY_STATS_QUERY_KEY })
+
+      // Confirmation toast. Cache updates are synchronous now, so without
+      // a toast a successful save is invisible (the field shows the new
+      // value but it's the same one the user just typed). Pick a label
+      // from the changed field; fall back to a count for multi-field
+      // patches and a generic message if neither applies.
+      const fieldLabels: Record<string, string> = {
+        filename: 'Filename updated',
+        rating: 'Rating updated',
+        color_label: 'Color updated',
+        user_title: 'Title updated',
+        user_notes: 'Notes updated',
+        taken_at: 'Date updated',
+      }
+      const fields = Object.keys(variables)
+      let detail: string
+      if (fields.length === 1) {
+        const k = fields[0]
+        if (k === 'is_discarded') {
+          detail = variables.is_discarded ? 'Discarded' : 'Restored'
+        } else {
+          detail = fieldLabels[k] ?? 'Updated'
+        }
+      } else if (fields.length > 1) {
+        detail = `${fields.length} fields updated`
+      } else {
+        detail = 'Updated'
+      }
+      toast.success('Saved', detail)
     },
   })