On-disk content and thumbnail cache with per-photo fetch and prefetch (closes #46)

Adds the on-disk content and thumbnail cache keyed by fileID: originals/ and thumbnails/ under cacheDirectory, present-means-complete (streaming atomic rename), orphan temp reaping on open. Photo.original/thumbnail return a cached path with no network when present, else fetch through the shared request pool; thumbnails.ensure drives the thumbnail pool with priority, dedup and AbortSignal. One shared RequestPools set serves both the ML fetch and the content cache. Content-hash integrity is deferred (#68); authenticated streaming decrypt guarantees integrity now.

Model: opus-4-8

src/client.ts

@@ -13,6 +13,10 @@ import {
     downloadFile as dlFile,
     downloadThumbnail as dlThumb,
 } from "./download/index.js";
+import {
+    makeDownloadContentSource,
+    type ContentSource,
+} from "./library/content.js";
 import type {
     Collection,
     EnteFile,
@@ -141,6 +145,14 @@ export class Client {
         return this.api;
     }
 
+    // The content-cache byte source over this client's API: each fetch is the
+    // download layer's request + streaming decrypt + atomic write. `Library`
+    // calls this to enable the on-disk content cache.
+    contentSource(): ContentSource {
+        this.assertLoggedIn();
+        return makeDownloadContentSource(this.api);
+    }
+
     private assertLoggedIn(): void {
         if (this.loggedOut) throw new Error("Client has been logged out");
     }