audit: wire pushback gate to hard merge gate and hash-chain ledger (gap 1)

Track 3 decisions (audit_pushback) now really block the server merge
endpoint and really land on the track 4 hash chain, via the new
cloud/audit_wiring.py translation layer:

- POST /tasks/{id}/merge ANDs the chain pushback block
  (merge_blocked/release_blocked/blocked_by=audit_pushback/round/
  failed_checks, replayed per module+contract version) with the
  hard gate's five checks in every gate mode; either side blocks
  -> 409, status unchanged, audit_pushback_blocked appended to
  merge_gate.blocks. Ledger reads that fail (other than a missing
  audit_ledger table) fail closed to 409.
- Every pushback/fix/re-audit/rebuild/escalation step is written
  to the ledger with track 3 fields authoritative and track 4
  storage authoritative; CONDITIONAL maps to fail at the boundary
  with the original preserved, history never rewritten.
- Hard-gate 409 blocks codes feed pushback packages machine-first
  (branch protection survives as its own extra check).
- Re-audit QA guard: a claimed pass with un-rerun or failed QA
  links never reaches the chain.
- Track 4 compat: one audit_wiring_event action for escalation /
  blocked-quota / audit-sample kinds, counted in the timeline.
- Resolved tension (documented in audit_wiring.py): a meta-review
  correction to fail re-blocks at once, but a correction to pass
  does NOT unblock by itself -- track 3 schedules a re-audit,
  it does not release the gate; track 4 failure counts still use
  effective verdicts, so the two views may disagree while a
  re-audit is pending.

Tests: cloud/tests/test_audit_wiring.py (18 items), incl. a real
HTTP end-to-end chain (fail -> 409 -> fix -> re-audit pass ->
200) and a separate QA-link chain test.

cloud/server.py

@@ -89,6 +89,7 @@ from cloud.peer_messages import row_get as _row_get
 from cloud.marker_tools import parse_spawn_defaults as _parse_spawn_defaults
 from cloud.merge_gate import evaluate_merge_gate as _merge_gate_evaluate
 from cloud.merge_gate import protection_from_env as _merge_gate_protection
+from cloud.audit_wiring import pushback_block_for_task as _pushback_block
 from cloud.merge_gate_hard import blocked_audit_detail as _gate_blocked_detail
 from cloud.merge_gate_hard import evaluate_hard_merge_gate as _merge_gate_hard
 from cloud.merge_gate_hard import hard_mode_from_env as _merge_gate_hard_mode
@@ -8087,6 +8088,18 @@ class CloudControlPlaneHandler(BaseHTTPRequestHandler):
         门禁阻断（任一模式）都写 merge_gate_blocked 审计后才回 409，
         这是第 3 轨自动推回的阻塞依据：推回包取响应 merge_gate 的
         blocks 码，不要解析审计文本；本接口不建推回任务、不改状态机。
+        审计推回接线（cloud/audit_wiring，第 1 项接线缺口）：五项
+        检查之外再与链上推回阻塞做“与”：链上任一模块仍阻塞
+        （merge_blocked）时，即使五项全过也 409、状态不变，并在
+        merge_gate.blocks 里追加 audit_pushback_blocked 码、响应
+        里附 audit_pushback 对象（merge_blocked / release_blocked
+        / blocked_by="audit_pushback" / round / failed_checks）。
+        推回阻塞与门禁模式无关：record 模式的显式确认只覆盖
+        保护风险，绝不覆盖推回阻塞；链读不到（audit_ledger 表
+        不存在）视为无推回状态，其它读链失败一律 fail-closed
+        409，不放行。注意：本接口没有独立的发布端点，
+        release_blocked 随响应返回、由未来的发布端点用同一
+        reader 强制；deploy.py 是运维脚本，不在本接口门禁内。
         本服务没有可信渠道直连 GitHub 核实 CI/保护：门禁强制的是
         “调用方提交的证据齐全且全绿”，谎报证据由审计与 PR/Checks
         只读核对覆盖，不在本接口的验证范围内。
@@ -8147,6 +8160,46 @@ class CloudControlPlaneHandler(BaseHTTPRequestHandler):
                 approved_exceptions=body.get("approved_exceptions") or (),
                 protection=protection, mode=mode,
                 explicit_confirm=bool(body.get("merge_gate_confirm")))
+        # 审计推回接线：链上阻塞与门禁五项为“与”关系，且与模式
+        # 无关。读链失败（表不存在除外，见 audit_wiring）时
+        # fail-closed：不证明无阻塞就不放行。
+        pushback = None
+        try:
+            conn = self._db()
+            try:
+                pushback = _pushback_block(conn, task_id)
+            finally:
+                conn.close()
+        except Exception:
+            pushback = None
+        if pushback is None:
+            try:
+                with _auth_store(self.server.db_path) as store:
+                    store.audit(
+                        user_id, task["project_id"],
+                        "merge_gate_blocked",
+                        "%s mode=%s blocks=audit_pushback_state_unavailable"
+                        % (task_id, gate.get("mode", "hard")))
+            except Exception:
+                pass
+            self._send_json(409, {
+                "error": "audit pushback state unavailable",
+                "merge_gate": gate,
+                "audit_pushback": {
+                    "merge_blocked": True,
+                    "release_blocked": True,
+                    "blocked_by": "audit_pushback_state_unavailable",
+                    "round": None,
+                    "failed_checks": [],
+                },
+            })
+            return
+        gate["audit_pushback"] = pushback
+        if pushback["merge_blocked"]:
+            gate["allowed"] = False
+            blocks = gate.setdefault("blocks", [])
+            if "audit_pushback_blocked" not in blocks:
+                blocks.append("audit_pushback_blocked")
         if not gate["allowed"]:
             with _auth_store(self.server.db_path) as store:
                 store.audit(user_id, task["project_id"],