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.

photo-api/.env.example

@@ -29,10 +29,19 @@ JWT_SECRET=
 # Public site origin, used to build share links and allow dev CORS
 FRONTEND_URL=https://spanglishcommunity.com
 
-# Photo storage. Local disk by default; setting BOTH S3_ENDPOINT and
-# S3_BUCKET switches to S3 (same convention as the backend's media storage).
-# Use a photos-specific bucket — do not reuse the backend's media bucket.
+# Photo storage. Which backend serves requests:
+#   auto  (default) S3 when S3_ENDPOINT + S3_BUCKET are set, local otherwise
+#         — the backend's convention (backend/src/lib/storage.ts)
+#   local always local disk, even with the S3 settings filled in
+#   s3    always S3
+# Keep both sides filled in and flip this one line to switch backends. Both
+# are also required by the library sync (`npm run sync:photos:to-s3` /
+# `:to-local`), which copies every photo from one backend to the other.
+STORAGE_BACKEND=auto
+
 STORAGE_PATH=./data/photos
+
+# Use a photos-specific bucket — do not reuse the backend's media bucket.
 #S3_ENDPOINT=
 #S3_REGION=auto
 #S3_BUCKET=spanglish-photos