feat: tamper-evident audit ledger (hash chain) + dual-column cost view

- Hash-chain ledger (cloud/ledger.py): prev_hash/entry_hash per record,
  SQLite append-only triggers, UNIQUE prev/entry hash with fail-closed
  retry at the transaction boundary; verify/export (incl. expected-head
  check for tail truncation); legacy audit_log rows marked pre-chain via
  a single ledger_genesis marker, never rewritten.
- AuthStore.audit() writes audit_log + chain entry in one transaction.
- Track-3 interface: audit_verdict / audit_pushback_round /
  audit_rebuild / third-failure assessment / test-change audit /
  meta-review / rebuild-notice entries; per-(module, contract-version)
  failure counts derived from the ledger (pass or completed rebuild
  resets; total > 3 triggers rebuild); acceptance-triggered rebuilds
  require a confirmed user notice.
- Admin read-only endpoints: GET /admin/audit-ledger/verify|export.
- Cost: dual_column_view (priced_usd_micros + billing note, wording
  reused verbatim from PR #212) wired into project/task cost endpoints;
  by-model and per-creator reporting stay in #212, not duplicated here.
- PG schema + migration lists updated for audit_ledger (incl. sequence
  setval alignment).

cloud/cost.py

@@ -135,6 +135,8 @@ def price_usd_micros(model, input_tokens, cached_input_tokens,
     return int(round(usd * 1000000))
 
 
+# 双列文案：逐字复用 PR #212（feat-cost-quota，5b30c61）的
+# BILLING_NOTE，本轨不拥有成本口径、不另造一套说法。
 BILLING_NOTE = (
     "价格表代理价（经 CLOUD_MODEL_PRICES_JSON / CLOUD_MODEL_PRICES_FILE"
     " 覆盖层），非中转实际账单；未定价 token 不计入美元、不等于免费。"
@@ -168,6 +170,24 @@ def quota_cutoff_reason(quota_usd_micros, used_usd_micros,
     return None
 
 
+def dual_column_view(report):
+    """双列视图：在已有 _report 结果上加定价列别名与账单说明。
+
+    不改 _report（旧单测按其键集断言）与旧键：usd_micros 仍是
+    有价行之和，unpriced_actual_tokens 仍是未定价 actual token；
+    本函数只加 priced_usd_micros（与 usd_micros 同值）与
+    billing_note。未定价模型分列（by_model / unpriced_models）
+    与按创建者口径是 PR #212 的范围，本轨不重复实现——#212
+    合并后本函数由其 dual_column_* 取代，端点改走 dual_column_*
+    （同名同值键，切换无语义差）；#212 未合并前，本视图是
+    任务/项目成本端点的双列最小实现。
+    """
+    view = dict(report)
+    view["priced_usd_micros"] = view["usd_micros"]
+    view["billing_note"] = BILLING_NOTE
+    return view
+
+
 def unpriced_token_cap():
     """Monthly unpriced-token ceiling for users who have a quota row.