fix(queue): enforce queued ⇒ no run-ownership invariant (ORCH-126)

Queued analyst-jobs hung forever even with ORCH_SERIAL_GATE_ENABLED=false
(incident ORCH-124/125, job 2286: queued + run_id=759/760 + pid=35/42 +
started_at=NULL — physically impossible). No path returning a job to
'queued' reset its run-ownership (run_id/pid); after a container restart a
reused pid made pid_alive(stale)=True, so the job-reaper Tier-1 saw a phantom
'running' and at max_concurrency=1 wedged the claim of the whole shared queue.

Enforce the invariant `status='queued' ⇒ run_id IS NULL AND pid IS NULL AND
started_at IS NULL` on existing columns (no schema change):

- D1 forward-cleanup: requeue_running_jobs / mark_job('queued') /
  mark_job_transient / reap_running_job('queued') reset run_id=NULL, pid=NULL
  in the same UPDATE that clears started_at; atomic status-guards preserved.
- D2 clean claim: claim_next_job resets pid/run_id on the queued->running flip
  (defense-in-depth) so the row carries pid IS NULL until _spawn stamps it.
- D4 self-heal + observability: db.find_impossible_queued_jobs /
  sanitize_impossible_queued run at startup (main.lifespan) and on each reaper
  tick (JobReaper.sanitize_impossible_queued_once, never-raise); counter
  impossible_queued_total in the GET /queue reaper block. Kill-switch
  ORCH_IMPOSSIBLE_QUEUED_SANITIZE_ENABLED (default on; gates only the D4 sweep).
- D5: reaper Tier-1 unchanged — the fix restores its precondition (pid reflects
  THIS run). Marked invariants ORCH-065/113/114/099 preserved.

Tests: tests/test_orch126_queued_stale_run.py (TC-01 mandatory regression
red->green; TC-02..TC-10). Full pytest tests/ -q green (2189 passed).
Docs: internals.md (run-ownership invariant section), .env.example, CHANGELOG;
cross-cutting adr-0052.

Refs: ORCH-126
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.task-dev.md

@@ -1,4 +1,4 @@
-Work item: ORCH-124
+Work item: ORCH-126
 Repo: orchestrator
-Branch: feature/ORCH-124-bug-serial-gate-treats-backlog
+Branch: feature/ORCH-126-bug-queued-job-can-keep-stale-
 Stage: development