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/dbtest/reset.go

@@ -109,6 +109,7 @@ var dataTables = []string{
 	"mb_release_groups",             // M502 #5519
 	"mb_artists",                    // M502 #5519
 	"requests",                      // M502 #5520 (was lidarr_requests)
+	"download_interventions",        // M502 #5523
 	"release_blocks",                // M502 #5521
 	"grabs",                         // M502 #5521
 	"want_candidates",               // M502 #5521
@@ -187,6 +188,7 @@ func ResetDB(t *testing.T, pool *pgxpool.Pool) {
 	if _, err := pool.Exec(ctx, `
 		UPDATE acquisition_settings
 		   SET auto_grab = DEFAULT, search_interval_hours = DEFAULT, allowed_qualities = DEFAULT, import_root = DEFAULT,
+		       seed_ratio = DEFAULT, seed_days = DEFAULT, stall_hours = DEFAULT,
 		       last_pass_at = NULL, last_pass = DEFAULT, updated_at = DEFAULT`,
 	); err != nil {
 		t.Fatalf("dbtest.ResetDB reset acquisition settings: %v", err)