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_log_redaction.py

@@ -44,12 +44,14 @@ def test_phone_numbers_inside_free_text_are_masked() -> None:
     assert "138****8000" in str(out["detail"])
 
 
+# Synthetic key. Never paste a live key here: this file is committed, so a real
+# key would land in git history permanently on the first commit.
+FAKE_API_KEY = "sk-" + "0" * 8 + "deadbeef" + "0" * 16
+
+
 def test_api_keys_in_free_text_are_masked() -> None:
-    # Never put a real key in a fixture, not even to prove it gets redacted:
-    # the test file itself would then carry the secret into git history.
-    fake_key = "sk-" + "0f1e2d3c4b5a69788796a5b4c3d2e1f0"
-    out = redact({"event": "cfg", "detail": f"using {fake_key}"})
-    assert fake_key not in str(out["detail"])
+    out = redact({"event": "cfg", "detail": f"using {FAKE_API_KEY}"})
+    assert FAKE_API_KEY not in str(out["detail"])
     assert REDACTED in str(out["detail"])