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>

src/config.py

@@ -1001,9 +1001,22 @@ class Settings(BaseSettings):
     #                                layer (env ORCH_SERIAL_GATE_FREEZE_ENABLED). False
     #                                -> freeze is neither set (post-deploy DEGRADED) nor
     #                                consulted in the claim gate.
+    #   serial_gate_pause_enabled -> ORCH-124 (adr-0051 D6): independent tumbler for
+    #                                the per-task "park" axis (env
+    #                                ORCH_SERIAL_GATE_PAUSE_ENABLED). True (default) ->
+    #                                a task with tasks.paused_at NOT NULL is excluded
+    #                                from the serial-gate "active task" predicate so an
+    #                                URGENT successor may overtake a paused predecessor.
+    #                                Default is a TRUE no-op until an operator pauses a
+    #                                task (paused_at is NULL for all rows). False ->
+    #                                pause-term omitted, serial-gate is byte-for-byte
+    #                                ORCH-088/090 (deliberate rollback). Scope reuses
+    #                                serial_gate_repos (no new *_repos flag); subordinate
+    #                                to the serial_gate_enabled kill-switch.
     serial_gate_enabled: bool = True
     serial_gate_repos: str = ""
     serial_gate_freeze_enabled: bool = True
+    serial_gate_pause_enabled: bool = True
 
     # ORCH-090: STOP-status task cancellation (stop active agent + full progress
     # reset) and the relaunch-hole close. A new logical Plane key `stop` (fail-closed,