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/library/store.ts

@@ -179,6 +179,16 @@ export class MetadataStore {
         return this.files.get(fileKey(collectionID, fileID));
     }
 
+    // Any membership of a file, or undefined. Every membership re-wraps the
+    // same underlying content key, so any one is enough to fetch the bytes;
+    // the content cache resolves a fileID to a file this way.
+    getFileByID(fileID: number): EnteFile | undefined {
+        for (const file of this.files.values()) {
+            if (file.id === fileID) return file;
+        }
+        return undefined;
+    }
+
     listFiles(collectionID: number): EnteFile[] {
         return [...this.files.values()].filter(
             (f) => f.collectionID === collectionID,