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/internal/storage/storage.go

@@ -7,6 +7,7 @@ package storage
 import (
 	"context"
 	"errors"
+	"fmt"
 	"io"
 	"time"
 
@@ -17,16 +18,30 @@ import (
 // callers then stream the object through the API instead.
 var ErrNoPresign = errors.New("presigned URLs not supported")
 
+// ErrNotExist is what Stat reports for a missing object on either backend.
+var ErrNotExist = errors.New("object does not exist")
+
 type Storage interface {
 	Put(ctx context.Context, key string, r io.Reader, size int64, contentType string) error
 	Open(ctx context.Context, key string) (io.ReadCloser, int64, error)
+	// Stat returns the object's size without reading it, or ErrNotExist.
+	Stat(ctx context.Context, key string) (int64, error)
 	Delete(ctx context.Context, key string) error
 	PresignGet(ctx context.Context, key, downloadFilename, contentType string, expiry time.Duration) (string, error)
 }
 
+// New builds the backend that serves requests (see config.S3Enabled).
 func New(cfg config.Config) (Storage, error) {
 	if cfg.S3Enabled() {
-		return newS3(cfg)
+		return NewS3(cfg)
 	}
-	return newLocal(cfg.StoragePath)
+	return NewLocal(cfg.StoragePath)
+}
+
+// Name describes a backend for log lines.
+func Name(cfg config.Config, s Storage) string {
+	if _, ok := s.(*s3Store); ok {
+		return fmt.Sprintf("S3 bucket %s at %s", cfg.S3Bucket, cfg.S3Endpoint)
+	}
+	return fmt.Sprintf("local disk at %s", cfg.StoragePath)
 }