Validate decoded photo size and require a photos row to serve

decodePhoto checked the GIF logical screen size, but gif.Decode returns
the first frame, which may be smaller; a 400x400 screen with a 1x1 first
frame passed and was scaled. The short-side check now measures the
decoded image's bounds. The pixel cap still comes from the header, before
decoding.

OpenPhoto served any file whose name was a canonical ULID and stored
size, with no photos row required. It now looks the id up with the new
GetPhoto query and reports an id with no row as fs.ErrNotExist, so
GET /media/photos/{id}-{size}.jpg is 404 for it even when a file exists.
OpenPhoto takes the request context for the lookup.

The media test fixture records a photos row for its served photo and
adds an orphan photo whose files exist without a row; both its sizes
must be 404.

Maestro task: 8a432108-d3ba-4013-ae8c-11963981d641

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/db/photos.sql.go

@@ -59,6 +59,28 @@ func (q *Queries) CreatePhoto(ctx context.Context, arg CreatePhotoParams) (Photo
 	return i, err
 }
 
+const getPhoto = `-- name: GetPhoto :one
+SELECT id, source_url, local_path, width, height, sha256, fetched_at, created_at, updated_at FROM photos
+WHERE id = ?
+`
+
+func (q *Queries) GetPhoto(ctx context.Context, id string) (Photo, error) {
+	row := q.db.QueryRowContext(ctx, getPhoto, id)
+	var i Photo
+	err := row.Scan(
+		&i.ID,
+		&i.SourceUrl,
+		&i.LocalPath,
+		&i.Width,
+		&i.Height,
+		&i.Sha256,
+		&i.FetchedAt,
+		&i.CreatedAt,
+		&i.UpdatedAt,
+	)
+	return i, err
+}
+
 const getPhotoBySHA256 = `-- name: GetPhotoBySHA256 :one
 SELECT id, source_url, local_path, width, height, sha256, fetched_at, created_at, updated_at FROM photos
 WHERE sha256 = ?

internal/db/querier.go

@@ -43,6 +43,7 @@ type Querier interface {
 	// Returns sql.ErrNoRows until the row exists.
 	GetIntegrationsGoogle(ctx context.Context) (IntegrationsGoogle, error)
 	GetJob(ctx context.Context, id string) (Job, error)
+	GetPhoto(ctx context.Context, id string) (Photo, error)
 	// Finds stored image bytes by content hash. The schema does not make sha256
 	// unique, so the earliest-created (lowest id) match is returned.
 	GetPhotoBySHA256(ctx context.Context, sha256 string) (Photo, error)