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.

README.md

@@ -86,8 +86,23 @@ npm run dev:photos      # go run ./cmd/photo-api
 npm run build:photos    # go build -o bin/photo-api
 npm run migrate:photos  # apply photos_* migrations
 npm run test:photos     # go test ./...
+
+# Move the photo library between storage backends (both must be configured
+# in photo-api/.env; the source is left untouched, reruns skip what is there)
+npm run sync:photos:to-s3      # local disk -> S3
+npm run sync:photos:to-local   # S3 -> local disk
+npm run sync:photos -- to-s3 --dry-run   # flags: --dry-run --overwrite
+                                         #        --concurrency=N --gallery=<id>
+
+# Uploads are deduplicated per gallery by content hash. Photos uploaded before
+# that existed need hashing once (idempotent, deletes nothing):
+npm run backfill:photos:checksums
 ```
 
+After a sync, set `STORAGE_BACKEND=s3` (or `local`) in `photo-api/.env` and
+restart the service to serve from the new backend. See
+[`photo-api/README.md`](photo-api/README.md#move-the-library-between-backends).
+
 You can also run per workspace:
 
 ```bash
@@ -117,6 +132,7 @@ Key settings (see `photo-api/.env.example`):
 - **DB**: `DB_TYPE` and `DATABASE_URL` — point at the **same** database as the backend
 - **Auth**: none needed for user auth — the service validates Better Auth session cookies against the shared database. `PHOTO_VIEW_SECRET` signs gallery image view tokens (falls back to `JWT_SECRET` during migration).
 - **Storage**: `STORAGE_PATH` (local disk) or `S3_ENDPOINT` + `S3_BUCKET` (S3/Garage/MinIO); S3 downloads use short-lived presigned URLs
+- **Storage switch**: `STORAGE_BACKEND=auto|local|s3` (`auto` = S3 when it is configured). Keep both sides configured and flip this one line to move between them; `npm run sync:photos:to-s3` / `:to-local` copies the existing library first
 - **Uploads/worker**: `MAX_UPLOAD_MB`, `WORKER_CONCURRENCY` (a worker generates thumb/preview JPEG variants with EXIF stripped)
 
 ### Frontend (`frontend/.env`)