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/library/content.ts

@@ -108,6 +108,11 @@ export interface ContentOptions {
 export interface PhotoContent {
     original(fileID: number, opts?: ContentOptions): Promise<ContentResult>;
     thumbnail(fileID: number, opts?: ContentOptions): Promise<ContentResult>;
+    // Where a backup stores the original, whether or not it is there yet. For
+    // a live photo not yet stored, it carries the title's extension, and the
+    // backup may store the image under a different one.
+    savePath(fileID: number): string | undefined;
+    isLocal(fileID: number): boolean;
 }
 
 export interface EnsureResult {
@@ -435,6 +440,31 @@ export class ContentCache implements PhotoContent, ThumbnailsAPI {
         return this.get(fileID, "thumbnail", "on-demand", opts?.onProgress);
     }
 
+    // Where a backup to the download directory stores the file's original,
+    // whether or not it is there yet: for a live photo already stored, its
+    // image. For a live photo not yet stored, it carries the title's
+    // extension, and the backup may store the image under a different one.
+    // Undefined with no download directory.
+    savePath(fileID: number): string | undefined {
+        const file = this.getFile(fileID);
+        if (this.downloadDirectory === undefined || file === undefined)
+            return undefined;
+        const dir = join(this.downloadDirectory, "originals");
+        return (
+            storedOriginal(dir, file)?.path ?? join(dir, nameInOriginals(file))
+        );
+    }
+
+    // Whether the whole original is in the download directory, as a backup
+    // stores it. A copy only in the cache does not count.
+    isLocal(fileID: number): boolean {
+        const file = this.getFile(fileID);
+        if (this.downloadDirectory === undefined || file === undefined)
+            return false;
+        const dir = join(this.downloadDirectory, "originals");
+        return storedOriginal(dir, file) !== undefined;
+    }
+
     // Get an original for a backup. One not present anywhere is written
     // straight to `destination` and recorded there, so no second copy lands
     // in the cache; one already present is returned where it is.