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/backup.ts

@@ -45,7 +45,7 @@ import {
 import { copyFile, rename, rm } from "node:fs/promises";
 import { basename, dirname, extname, join, relative } from "node:path";
 
-import { fsyncPath } from "./download/index.js";
+import { fsyncPath, removeLeftoverTempFiles } from "./download/index.js";
 import { safeExtension, sanitizeFileName } from "./filename.js";
 import type { Collection, EnteFile } from "./model/types.js";
 
@@ -184,36 +184,6 @@ const copyAtomic = async (src: string, dest: string): Promise<void> => {
     }
 };
 
-// A process-ID check: signal 0 delivers nothing and only reports whether the
-// process exists. EPERM means it exists but belongs to another user.
-const isRunning = (pid: number): boolean => {
-    try {
-        process.kill(pid, 0);
-        return true;
-    } catch (err) {
-        return (err as NodeJS.ErrnoException).code === "EPERM";
-    }
-};
-
-// Delete the temp files `copyAtomic` leaves behind when a backup is killed
-// before its rename. Only files whose process is no longer running are
-// removed, so a backup running at the same time keeps its own. A reused
-// process ID can only keep a leftover a while longer, never remove a live one.
-const removeLeftoverTempFiles = (dir: string): void => {
-    let names: string[];
-    try {
-        names = readdirSync(dir);
-    } catch {
-        return;
-    }
-    for (const name of names) {
-        const match = /^\.quak-backup-.*-(\d+)-[0-9a-z]*\.tmp$/.exec(name);
-        if (match && !isRunning(Number(match[1]))) {
-            rmSync(join(dir, name), { force: true });
-        }
-    }
-};
-
 // Ensure `linkPath` is a symlink to `target`, rebuilding a missing, wrong, or
 // non-symlink entry. Throws on failure (a directory in the way, no permission)
 // so the caller records it and moves on rather than aborting the run.