fix(files): a file is added when it is done, not when it was asked for

A download that failed for days and then went through sat days down the library, under its
request date, and read as missing. A job now records when it became done (`ready_at`), stamped
where the expiry already is (`folders.stamp_done`, `finalize_if_running`) and on an insert that is
already done; a check constraint holds every path to done to it. The library sorts and dates its
"Added" by it; the queue keeps the request order.

The migration dates a file from before by its "ready to watch" notice where the inbox still holds
one, else by its request date, where it already sat.

frontend/src/lib/api/downloads.ts

@@ -94,7 +94,8 @@ export interface DownloadJob {
   error_code: string | null; // failure category, see backend downloads.errors.classify
   error_detail: string | null; // host-specific failure-catalog entry, see downloads.errors.diagnose
   queue_pos: number;
-  created_at: string | null;
+  created_at: string | null; // when it was asked for
+  ready_at?: string | null; // when it became done — the library's "Added"
   source_kind: string;
   source_ref: string;
   source_url: string | null; // clean "downloaded from" URL (null for edit clips)