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/s3.go

@@ -2,14 +2,19 @@ package storage
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"io"
+	"net/http"
 	"time"
 
 	"github.com/aws/aws-sdk-go-v2/aws"
+	awshttp "github.com/aws/aws-sdk-go-v2/aws/transport/http"
 	awsconfig "github.com/aws/aws-sdk-go-v2/config"
 	"github.com/aws/aws-sdk-go-v2/credentials"
 	"github.com/aws/aws-sdk-go-v2/service/s3"
+	"github.com/aws/aws-sdk-go-v2/service/s3/types"
+	"github.com/aws/smithy-go"
 
 	"git.azzamo.net/Michilis/Spanglish/photo-api/internal/config"
 )
@@ -23,6 +28,12 @@ type s3Store struct {
 	bucket  string
 }
 
+// NewS3 builds the S3 backend explicitly, whichever backend is active —
+// `photo-api sync` needs both sides at once.
+func NewS3(cfg config.Config) (Storage, error) {
+	return newS3(cfg)
+}
+
 func newS3(cfg config.Config) (*s3Store, error) {
 	awsCfg, err := awsconfig.LoadDefaultConfig(context.Background(),
 		awsconfig.WithRegion(cfg.S3Region),
@@ -65,6 +76,28 @@ func (s *s3Store) Open(ctx context.Context, key string) (io.ReadCloser, int64, e
 	return out.Body, aws.ToInt64(out.ContentLength), nil
 }
 
+func (s *s3Store) Stat(ctx context.Context, key string) (int64, error) {
+	out, err := s.client.HeadObject(ctx, &s3.HeadObjectInput{
+		Bucket: aws.String(s.bucket),
+		Key:    aws.String(key),
+	})
+	if err != nil {
+		// Compatibles differ: some answer NotFound, some NoSuchKey, and a
+		// HEAD carries no body to parse, so fall back to the status code.
+		var notFound *types.NotFound
+		var noKey *types.NoSuchKey
+		var apiErr smithy.APIError
+		var respErr *awshttp.ResponseError
+		if errors.As(err, &notFound) || errors.As(err, &noKey) ||
+			(errors.As(err, &apiErr) && (apiErr.ErrorCode() == "NotFound" || apiErr.ErrorCode() == "NoSuchKey")) ||
+			(errors.As(err, &respErr) && respErr.HTTPStatusCode() == http.StatusNotFound) {
+			return 0, ErrNotExist
+		}
+		return 0, err
+	}
+	return aws.ToInt64(out.ContentLength), nil
+}
+
 func (s *s3Store) Delete(ctx context.Context, key string) error {
 	_, err := s.client.DeleteObject(ctx, &s3.DeleteObjectInput{
 		Bucket: aws.String(s.bucket),