Keep another process's downloads when opening a library (closes #105)

The download writer's temp files are now named .quak-<pid>-<random>.tmp.
removeLeftoverTempFiles moves from the backup into the download module and
deletes a .quak-*.tmp file only when the process ID in its name is no
longer running; the content cache calls it at open() instead of deleting
every temp file, so a download in progress in another process sharing the
cache survives. The README backup layout and TODO.md are updated.

Model: opus-5-5

src/library/content.ts

@@ -39,6 +39,7 @@ import {
     downloadFile,
     downloadThumbnail,
     type ProgressCallback,
+    removeLeftoverTempFiles,
 } from "../download/index.js";
 import { safeExtension } from "../filename.js";
 import type { EnteFile } from "../model/types.js";
@@ -46,8 +47,6 @@ import type { Priority, RequestPools } from "./pools.js";
 
 const DIR_MODE = 0o700;
 const FILE_MODE = 0o600;
-const TEMP_PREFIX = ".quak-";
-const TEMP_SUFFIX = ".tmp";
 const GIB = 1024 * 1024 * 1024;
 // Owner ruling (#36): bound the originals cache at 100 GiB, but back off when
 // the volume has under 50 GiB free so the cache never crowds the disk.
@@ -654,6 +653,9 @@ export class ContentCache implements PhotoContent, ThumbnailsAPI {
     }
 
     private async scan(dir: string, into: Map<number, string>): Promise<void> {
+        // Another process sharing this cache may still be writing its temp
+        // files, so only those whose process has exited are removed.
+        removeLeftoverTempFiles(dir);
         let entries: string[];
         try {
             entries = await readdir(dir);
@@ -661,12 +663,6 @@ export class ContentCache implements PhotoContent, ThumbnailsAPI {
             return;
         }
         for (const name of entries) {
-            if (name.startsWith(TEMP_PREFIX) && name.endsWith(TEMP_SUFFIX)) {
-                await rm(join(dir, name), { force: true }).catch(
-                    () => undefined,
-                );
-                continue;
-            }
             const id = fileIDFromName(name);
             const path = join(dir, name);
             if (id !== undefined && existsSync(path)) into.set(id, path);