Add photo content-hash dedup and local↔S3 library sync.

Uploads skip per-gallery duplicates, checksums can be backfilled, and STORAGE_BACKEND plus sync tooling make switching storage backends safe.

frontend/src/app/admin/photos/[id]/page.tsx

@@ -18,6 +18,7 @@ import {
   CheckCircleIcon,
   ChevronDownIcon,
   ChevronUpIcon,
+  DocumentDuplicateIcon,
   ExclamationCircleIcon,
   ExclamationTriangleIcon,
   LinkIcon,
@@ -34,7 +35,9 @@ interface UploadItem {
   name: string;
   sizeBytes: number;
   progress: number; // 0..1 while uploading
-  status: 'queued' | 'uploading' | 'processing' | 'error';
+  // 'duplicate' is terminal: the gallery already held these bytes, so nothing
+  // was stored and no new tile appears in the grid.
+  status: 'queued' | 'uploading' | 'processing' | 'error' | 'duplicate';
   error?: string;
   photoId?: string;
 }
@@ -117,8 +120,19 @@ export default function AdminGalleryDetailPage() {
         const { photos: added } = await photosApi.uploadPhotoWithProgress(id, file, (fraction) =>
           patchUpload(key, { progress: fraction })
         );
-        setPhotos((prev) => [...prev, ...added]);
-        patchUpload(key, { status: 'processing', progress: 1, photoId: added[0]?.id });
+        // A duplicate echoes back a photo already in the grid, so merge by id
+        // instead of appending (`duplicate` is an upload outcome, not photo
+        // state, so it is dropped here).
+        setPhotos((prev) => {
+          const byId = new Map(prev.map((p) => [p.id, p]));
+          added.forEach(({ duplicate: _duplicate, ...photo }) => byId.set(photo.id, photo));
+          return Array.from(byId.values());
+        });
+        patchUpload(key, {
+          status: added[0]?.duplicate ? 'duplicate' : 'processing',
+          progress: 1,
+          photoId: added[0]?.id,
+        });
       } catch (err) {
         patchUpload(key, {
           status: 'error',
@@ -148,7 +162,8 @@ export default function AdminGalleryDetailPage() {
   };
 
   // A row is "done" once its photo finished processing; the panel derives
-  // this from the photos list instead of tracking it separately.
+  // this from the photos list instead of tracking it separately. 'duplicate'
+  // is terminal and never reconciled — its photo was already there.
   const displayStatus = (u: UploadItem): { state: string; error?: string } => {
     if (u.status === 'processing' && u.photoId) {
       const photo = photos.find((p) => p.id === u.photoId);
@@ -579,6 +594,11 @@ export default function AdminGalleryDetailPage() {
                             {ds.state === 'queued' && (es ? 'En cola' : 'Queued')}
                             {ds.state === 'processing' && (es ? 'Procesando…' : 'Processing…')}
                             {ds.state === 'done' && formatBytes(u.sizeBytes)}
+                            {ds.state === 'duplicate' && (
+                              <span className="text-amber-600">
+                                {es ? 'Ya está en esta galería' : 'Already in this gallery'}
+                              </span>
+                            )}
                             {ds.state === 'error' && (
                               <span className="text-red-600" title={ds.error}>
                                 {ds.error}
@@ -588,6 +608,9 @@ export default function AdminGalleryDetailPage() {
                         </div>
                         <span className="shrink-0">
                           {ds.state === 'done' && <CheckCircleIcon className="w-6 h-6 text-green-600" />}
+                          {ds.state === 'duplicate' && (
+                            <DocumentDuplicateIcon className="w-6 h-6 text-amber-600" />
+                          )}
                           {ds.state === 'error' && <ExclamationCircleIcon className="w-6 h-6 text-red-600" />}
                           {ds.state === 'processing' && (
                             <div className="animate-spin w-5 h-5 border-2 border-primary-yellow border-t-transparent rounded-full" />

frontend/src/lib/api/photos.ts

@@ -52,6 +52,9 @@ export interface Photo {
     preview?: string;
     original: string;
   };
+  // Upload responses only: these bytes were already in the gallery, so nothing
+  // was stored and the rest of this object describes the existing photo.
+  duplicate?: boolean;
 }
 
 export interface CreateGalleryInput {