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/Makefile

@@ -1,11 +1,11 @@
-.PHONY: start build test migrate clean help
+.PHONY: start build test migrate sync-to-s3 sync-to-local backfill-checksums clean help
 
 BINARY := bin/photo-api
 CMD    := ./cmd/photo-api
 
 help: ## Show available targets
 	@grep -E '^[a-zA-Z_-]+:.*?##' $(MAKEFILE_LIST) | \
-		awk 'BEGIN {FS = ":.*?## "}; {printf "  %-12s %s\n", $$1, $$2}'
+		awk 'BEGIN {FS = ":.*?## "}; {printf "  %-19s %s\n", $$1, $$2}'
 
 start: ## Run the photo-api server (go run)
 	go run $(CMD)
@@ -19,5 +19,14 @@ test: ## Run all Go tests
 migrate: ## Apply pending photos_* migrations
 	go run $(CMD) migrate
 
+sync-to-s3: ## Copy the photo library from local disk to S3 (needs both in .env)
+	go run $(CMD) sync to-s3
+
+sync-to-local: ## Copy the photo library from S3 back to local disk
+	go run $(CMD) sync to-local
+
+backfill-checksums: ## Hash photos uploaded before duplicate detection existed
+	go run $(CMD) backfill-checksums
+
 clean: ## Remove built binary
 	rm -rf bin