fix(backup): compressed -Fc dumps + pg_restore; reconcile subprocess timeouts (#739)

DB backup polish (plan-task #764 Q3):
- pg_dump now uses custom format (-Fc): compressed (much smaller on NFS) and
  restored via pg_restore. Artifact extension .sql → .dump; restore_db swaps
  psql -f for pg_restore -d. BackupRun.sql_path field name kept (it's just the
  db artifact path).
- Reconcile the subprocess guardrails: the DB timeout was 720s with a stale
  'Celery soft is 10 min' comment, but backup_db_task's soft limit is actually
  1800s — so the bounded-kill fired 18 min early. Set DB=1700s / images=21000s,
  each just under its task's Celery soft limit so _run_bounded stays the
  primary guard (an NFS D-state hang defeats Celery's own SIGKILL).

Real shrink of the DB is the #764 prune; this makes each dump smaller/faster
on top of that.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_tasks_backup.py

@@ -93,7 +93,7 @@ async def test_backup_db_task_creates_backup_run_row_status_ok(db_sync):
     ).one()
     assert row.kind == "db"
     assert row.status == "ok"
-    assert row.sql_path and row.sql_path.endswith(".sql")
+    assert row.sql_path and row.sql_path.endswith(".dump")
     assert row.size_bytes is not None and row.size_bytes > 0
     assert row.finished_at is not None
     assert row.error is None