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

TODO.md

@@ -25,6 +25,16 @@ declares one.
 
 # Completed Steps
 
+- 2026-10-01: A `Photo` has `savePath`, `isLocal`, `content()`, `exif()`,
+  `modifiedAt`, `hash` and `year` (issue 141). `savePath` is where
+  `lib.backup()` writes the original under the library's download directory; for
+  a live photo not yet stored, it carries the title's extension, and the backup
+  may store the image under a different one. `isLocal` says whether the whole
+  original is there. Both look only at the disk. `content()` returns the
+  original's bytes and `exif()` the common EXIF fields of a JPEG; both may
+  download the original, and `exif()` downloads no video. `PhotoRecord` gains
+  `modifiedAt` and `hash`, and the JPEG EXIF scan moved to `src/exif.ts`.
+
 - 2026-09-29: The "Workflow" list at the top of this file now says to branch
   from `next` and open a pull request that targets `next`, that the repository
   manager squash-merges a reviewed pull request into `next`, and that only sneak