fix(llm): price by call time and stop paying for thinking tokens

Two defects that only surface against the real DeepSeek API, both of which
break the cost model rather than the code:

deepseek-v4-flash enables thinking mode by default. The same single-turn
question took 13.5s with it on and 1.8s with it off, and the thinking tokens
are billed at the output rate. That misses NFR-PERF-02b (15s per turn) and
inflates every conversational call. SceneConfig now carries an explicit
`thinking` flag, all four V1 scenes set it to false, and the provider always
sends the parameter rather than relying on the upstream default. Reasoning
tokens are recorded separately in Usage.tokens_reasoning so the cost of ever
turning it back on is visible.

DeepSeek moves to peak/off-peak pricing on 2026-08-17, with peak (Beijing
09:00-12:00 and 14:00-18:00) at twice the off-peak rate. Reports are generated
mostly during peak hours, so a single blended rate would leave ob_ai_call --
the only input to V2 pricing -- off by nearly a factor of two.
PeakAwareRateTable resolves the rate at call time; cost is computed and stored
then, never recomputed later at whatever the current rate happens to be.

Also here:
- Replace the live API key in the redaction test with a synthetic one. The
  test that proves we redact secrets was itself a secret leak waiting for its
  first commit.
- Correct LLM_COST_RATES_JSON, which held placeholder rates.
- Make ids.SnowflakeGenerator take an injectable clock so rollover and
  clock-rollback are testable without patching a slots class.
- Rename exceptions to the -Error suffix (N818) and clear the remaining ruff,
  mypy and import-linter findings, including two dead lines: a duplicate
  pytestmark assignment and a no-op assert on a None return.

Gates: ruff, mypy (96 files incl. tests), 4/4 import contracts,
detect-secrets, 166 unit + 3 integration tests, migration up/down round-trip.

tests/unit/test_peak_pricing.py

@@ -0,0 +1,86 @@
+"""峰谷定价（DeepSeek 自 2026-08-17 起高峰单价翻倍）。
+
+保护的是：同一次调用在高峰与空闲时段必须记出不同的成本。若只配一套单价，
+``ob_ai_call`` 会与真实账单差出接近 2 倍，而这张表是 V2 定价的唯一依据。
+"""
+
+from __future__ import annotations
+
+from datetime import UTC, datetime, timedelta, timezone
+from decimal import Decimal
+
+import pytest
+
+from packages.llm.cost import (
+    ModelRate,
+    PeakAwareRateTable,
+    RateTable,
+    compute_cost_cents,
+    is_peak,
+)
+from packages.llm.types import Usage
+
+BEIJING = timezone(timedelta(hours=8))
+
+OFF_PEAK = ModelRate(Decimal("150"), Decimal("5"), Decimal("450"))
+PEAK = ModelRate(Decimal("300"), Decimal("10"), Decimal("900"))
+MODEL = "deepseek-v4-flash"
+
+
+def _table(*, enabled: bool = True) -> PeakAwareRateTable:
+    return PeakAwareRateTable(
+        RateTable({MODEL: OFF_PEAK}, default=OFF_PEAK),
+        RateTable({MODEL: PEAK}, default=PEAK),
+        enabled=enabled,
+    )
+
+
+@pytest.mark.parametrize(
+    ("hour", "expected"),
+    [
+        (0, False),
+        (8, False),
+        (9, True),  # 高峰左闭
+        (11, True),
+        (12, False),  # 高峰右开
+        (13, False),
+        (14, True),
+        (17, True),
+        (18, False),
+        (23, False),
+    ],
+)
+def test_peak_windows_follow_the_published_schedule(hour: int, expected: bool) -> None:
+    assert is_peak(datetime(2026, 8, 20, hour, 30, tzinfo=BEIJING)) is expected
+
+
+def test_utc_input_is_converted_to_beijing_before_judging() -> None:
+    # 02:00 UTC = 10:00 北京时间，属高峰。按 UTC 判定会把高峰算成空闲、成本少记一半
+    assert is_peak(datetime(2026, 8, 20, 2, 0, tzinfo=UTC)) is True
+
+
+def test_peak_rate_is_double_the_off_peak_rate() -> None:
+    table = _table()
+    usage = Usage(tokens_in=10_000, tokens_out=2_000, tokens_cached=0)
+
+    off_peak_cost = compute_cost_cents(
+        usage, table.for_model(MODEL, datetime(2026, 8, 20, 13, 0, tzinfo=BEIJING))
+    )
+    peak_cost = compute_cost_cents(
+        usage, table.for_model(MODEL, datetime(2026, 8, 20, 10, 0, tzinfo=BEIJING))
+    )
+
+    assert peak_cost == off_peak_cost * 2
+
+
+def test_disabling_peak_pricing_keeps_one_rate() -> None:
+    # 供应商若取消峰谷定价，改一个开关即可，不必回滚代码
+    table = _table(enabled=False)
+    assert table.for_model(MODEL, datetime(2026, 8, 20, 10, 0, tzinfo=BEIJING)) is OFF_PEAK
+
+
+def test_unknown_model_still_costs_money_in_both_tiers() -> None:
+    table = _table()
+    for hour in (10, 13):
+        rate = table.for_model("some-new-model", datetime(2026, 8, 20, hour, tzinfo=BEIJING))
+        assert rate.input_cents_per_mtok > 0