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/local.go

@@ -17,6 +17,12 @@ type local struct {
 	root string
 }
 
+// NewLocal builds the local-disk backend explicitly, whichever backend is
+// active — `photo-api sync` needs both sides at once.
+func NewLocal(root string) (Storage, error) {
+	return newLocal(root)
+}
+
 func newLocal(root string) (*local, error) {
 	if err := os.MkdirAll(root, 0o755); err != nil {
 		return nil, fmt.Errorf("create storage dir %s: %w", root, err)
@@ -74,6 +80,21 @@ func (l *local) Open(_ context.Context, key string) (io.ReadCloser, int64, error
 	return f, info.Size(), nil
 }
 
+func (l *local) Stat(_ context.Context, key string) (int64, error) {
+	p, err := l.path(key)
+	if err != nil {
+		return 0, err
+	}
+	info, err := os.Stat(p)
+	if os.IsNotExist(err) {
+		return 0, ErrNotExist
+	}
+	if err != nil {
+		return 0, err
+	}
+	return info.Size(), nil
+}
+
 func (l *local) Delete(_ context.Context, key string) error {
 	p, err := l.path(key)
 	if err != nil {

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),

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)
 }