fix(stage-engine): durable transition-ownership lease + expected-stage CAS (ORCH-114)

Close the root class of the ORCH-110/111/112/113 incident chain: side-effectful
stage transitions had no single ownership. `advance_stage` is re-enterable and wrote
the stage with a bare `UPDATE ... WHERE id=?` (no compare-and-swap), while >=5 actors
(monitor / Plane-webhook / reconciler F-1 / job-reaper / deploy-finalizer) enter the
same transition independently. A concurrent or post-restart re-entry therefore
re-applied irreversible effects (merge_pr / coverage-ratchet / image-rebuild /
prod-deploy initiation) and produced a contradictory rollback<->done (incident
ORCH-111, job 1914 / PR #130).

Two complementary layers, both additive, under one kill-switch, never-raise:
  1. Durable transition-lease (new table `transition_lease`) — owner-exclusion on
     ENTRY to the side-effectful region: a second actor that sees a LIVE owner does
     not start the heavy sub-gates at all (prevention, not post-hoc repair).
  2. Expected-stage CAS (`db.update_task_stage_cas`) — atomicity on the stage WRITE:
     a lost race aborts with NO side effect. Also closes the 6 paths that write the
     stage in bypass of advance_stage (gitea x5 + plane rollback).

Owner liveness = owner_pid + owner_boot_id (NOT a heartbeat — a blocking 900s merge
re-test cannot beat one; ADR-001 D3), making restart recovery free (a fresh boot_id
renders every prior lease stale -> reclaimed by recover_on_startup). The lease has no
own TTL: its hard age ceiling is the reaper Tier-3 backstop reaper_max_running_s, so
the cross-cutting budget invariant ORCH-065/109/110/113 is untouched.

Generalises ORCH-113 finalizer-liveness (process-local, Tier-2, deploy-staging) to a
durable cross-path lease: the reaper consults it on all relevant paths (defer live,
reclaim dead; Tier-3 ignores the marker -> bounded; a reap force-releases the lease);
reconciler F-1 and the Plane webhook defer on an active lease; main.lifespan calls
recover_on_startup() after requeue_running_jobs. finalizer_liveness.py is unchanged
(it remains the kill-switch-off fallback).

Scope self-hosting (transition_lease_repos="" -> orchestrator only; enduro untouched).
Kill-switch ORCH_TRANSITION_LEASE_ENABLED=false -> CAS degenerates to the prior
unconditional update_task_stage, lease inert, reaper -> ORCH-113 fallback (byte-for-
byte pre-ORCH-114). STAGE_TRANSITIONS / QG_CHECKS / check_* / machine-verdict keys /
existing table schemas — byte-for-byte (one additive table, no epoch column on tasks).

Observability: read-only `transition_lease` block in GET /queue + a Telegram alert on
forced/stale reclaim + optional POST /transition-lease/release?work_item=<id>.

Coverage: tests/test_orch114_transition_ownership.py (TC-01 mandatory regression of
the ORCH-111 class — red before fix, green after; TC-02..TC-14). Full suite green
(2048 passed); the 4 webhook tests that spied on the removed gitea.update_task_stage
were updated to spy on the new commit_stage_cas write path.

ADR: docs/work-items/ORCH-114/06-adr/ADR-001-transition-ownership-lease-and-stage-cas.md
Cross-cutting: docs/architecture/adr/adr-0045-transition-ownership-lease-and-stage-cas.md

Refs: ORCH-114
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/conftest.py

@@ -133,3 +133,28 @@ def _disable_merge_verify(monkeypatch):
         _cfg.settings, "merge_verify_autocreate_pr_enabled", False, raising=False
     )
     yield
+
+
+@pytest.fixture(autouse=True)
+def _disable_transition_lease(monkeypatch):
+    """ORCH-114: disable the transition-ownership lease + expected-stage CAS by
+    default in ALL tests.
+
+    The prod default is ON for the self-hosting repo (``transition_lease_enabled=True``,
+    ``transition_lease_repos=""`` -> orchestrator only). Left ON, the expected-stage
+    CAS (``update_task_stage_cas``) would change the stage-write semantics for every
+    existing test that calls ``advance_stage`` / the gitea-plane webhook handlers with
+    repo ``orchestrator`` (a CAS write needs the task row to actually BE at the
+    expected stage; the bare ``update_task_stage`` did not). We therefore default the
+    kill-switch OFF for the whole suite (mirrors ``_disable_merge_verify`` /
+    ``_disable_*`` precedent), which makes ``commit_stage_cas`` degenerate to the prior
+    unconditional ``update_task_stage`` and the lease inert -> the existing 2000+ tests
+    stay byte-for-byte (AC-9). The dedicated ORCH-114 test module
+    (``test_orch114_transition_ownership.py``) re-enables it via its own monkeypatch,
+    scoping the feature ON to just those tests.
+    """
+    from src import config as _cfg
+    monkeypatch.setattr(
+        _cfg.settings, "transition_lease_enabled", False, raising=False
+    )
+    yield