Photo: save path, is-local, content bytes, metadata and EXIF getters (closes #141)

`Photo` gains:

- `savePath` and `isLocal`: synchronous, disk only. Where `lib.backup()` writes the original under the library's `downloadDirectory`, and whether all of it is there; a copy only in the cache does not count.
- `content()` and `exif()`: async, may download. `exif()` reads the common EXIF fields of a JPEG and returns `{}` for anything else.
- The getters `modifiedAt` and `hash`, also on `PhotoRecord`, and `year`.

For a live photo the backup has not stored yet, `savePath` carries the title's extension, and the image may be stored under a different one. The JPEG EXIF scan moved to `src/exif.ts`. The exported `PhotoContent` interface gains `savePath` and `isLocal`.

Judgement call: `iso` is read only when the file stores it as a single number.

Model: opus-5-5

src/index.ts

@@ -84,6 +84,7 @@ export type {
     LibrarySnapshot,
     LibraryChange,
 } from "./library/records.js";
+export type { PhotoExif } from "./exif.js";
 export { decryptCollection, decryptFile } from "./model/index.js";
 export { downloadFile, downloadThumbnail } from "./download/index.js";
 export type {