fix(serial-gate): pause-without-blocking via per-task park signal (ORCH-124)

Fixes incident ORCH-116/ORCH-123: serial_gate defined a repo's "active task"
purely by machine stage (tasks.stage NOT IN ('done','cancelled')). Plane statuses
Backlog/Blocked/Needs-Input (layer-B indication, ORCH-066) do NOT change
tasks.stage (layer A), so a paused predecessor was indistinguishable from an active
one and held the FIFO gate closed against an urgent successor — the urgent fix
could not start until the paused task was formally done.

Introduces an explicit, durable, DB-resolvable per-task "park" signal — additive
nullable column tasks.paused_at (pattern of cancelled_at/track) — and a new
ORTHOGONAL scheduler "pause" axis. The serial-gate "active task" predicate becomes
`stage NOT IN ('done','cancelled') AND paused_at IS NULL` across all three points
(build_claim_clause / repo_has_active_task / _per_repo_snapshot). The terminal set
{done,cancelled} in serial_gate/task_deps/stages.py is byte-for-byte unchanged
(adr-0026 not regressed): task_deps/stages.py do NOT read paused_at, so a paused
declared dependency and an active repo_freeze STILL block (pause never bypasses
them — different axes). Anti-stale-base on resume relies on the existing deferred
branch cut (ORCH-088) + pre-merge auto_rebase_onto_main + merge-gate re-test
(ORCH-026/093/110) — no new rebase machinery.

Additive, under an independent sub-flag, never-raise, restart-safe; hot-claim
fail-OPEN and freeze fail-CLOSED preserved. STAGE_TRANSITIONS / QG_CHECKS / check_*
/ machine-verdict keys / existing table schemas are byte-for-byte untouched (this is
a queue-scheduler + observability change, not a Quality Gate).

- src/db.py: additive tasks.paused_at column (_ensure_column) + set/clear/is helpers
- src/serial_gate.py: _pause_layer_enabled() + pause-term in the 3 points; `paused`
  list + per-job `reason` (freeze>dependency>active-task>null) in the /queue snapshot
- src/config.py + .env.example: serial_gate_pause_enabled (default True = true no-op)
- src/main.py: POST /serial-gate/pause|resume?work_item=<id> (by образцу unfreeze)
- tests/test_orch124_serial_gate_pause.py: TC-01 mandatory incident regress + TC-02..15
- CHANGELOG.md: [Unreleased] entry

ADR: docs/work-items/ORCH-124/06-adr/ADR-001-serial-gate-pause-without-blocking.md
Cross-cutting: docs/architecture/adr/adr-0051-serial-gate-pause-without-blocking.md

Refs: ORCH-124

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

.task-dev.md

@@ -1,4 +1,4 @@
-Work item: ORCH-116
+Work item: ORCH-124
 Repo: orchestrator
-Branch: feature/ORCH-116-orch-replace-llm-tester-with-d
+Branch: feature/ORCH-124-bug-serial-gate-treats-backlog
 Stage: development