feat: download lifecycle — stalled and failed downloads are given up and searched again; imported ones seed, then go (M502 #5523)

Ported from Fabled Librarian's System 104. A new lifecycle poller, every
ten minutes:
- a download the client fails is given up at once;
- one nobody shares, or still fetching its file list, past the stall
  setting (6 h) is asked for again three times, then given up;
- giving up removes the download with its files, holds the release back
  for a week (release_blocks.expires_at), ends the grab and searches the
  album again, with no limit per album;
- a running download the client stops listing for half an hour ends and is
  searched again, not held back; an empty list is not taken as gone;
- an imported download seeds to ratio 2.0 or 14 days, then is removed;
- a download the client would not remove is asked again every pass.

Every condition and act is one open row per (client, download, condition)
in download_interventions. Activity shows what is wrong with a download and
how often it was retried, why one ended, its seeding, and a banner for a
client that does not answer. A running download can be given up by hand
(set aside for good). The seed ratio, seed days and stall hours are on the
search settings card. The download client lookup moves to
acquire.DownloadClientFor, shared by the importer and the lifecycle.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/db/migrations/0084_download_lifecycle.down.sql

@@ -0,0 +1,23 @@
+ALTER TABLE acquisition_settings
+    DROP COLUMN seed_ratio,
+    DROP COLUMN seed_days,
+    DROP COLUMN stall_hours;
+
+ALTER TABLE release_blocks DROP COLUMN expires_at;
+DELETE FROM release_blocks WHERE reason = 'stalled';
+ALTER TABLE release_blocks DROP CONSTRAINT release_blocks_reason_check;
+ALTER TABLE release_blocks ADD CONSTRAINT release_blocks_reason_check
+    CHECK (reason IN ('operator', 'files', 'import'));
+
+DROP INDEX grabs_seeding_idx;
+ALTER TABLE grabs
+    DROP COLUMN seeding,
+    DROP COLUMN ratio,
+    DROP COLUMN seed_released_at;
+
+UPDATE grabs SET end_reason = 'superseded' WHERE end_reason IN ('stalled', 'failed', 'abandoned', 'gone');
+ALTER TABLE grabs DROP CONSTRAINT grabs_end_reason_check;
+ALTER TABLE grabs ADD CONSTRAINT grabs_end_reason_check
+    CHECK (end_reason IN ('', 'superseded', 'imported', 'refused', 'held'));
+
+DROP TABLE download_interventions;