feat(cost): persist reasoning tokens and the peak/off-peak tier

ob_ai_call is the only input to V2 pricing, and it was dropping the two
numbers needed to interpret its own cost column.

tokens_reasoning was captured from the provider response but never written to
the table, so the cost of thinking mode was invisible once it left the
process. Folding it into tokens_out was not an option either -- it is billed
at the output rate, and mixing the two hides exactly the difference we now
have to watch.

rate_tier records which price list applied. Without it a row's cost_cents
cannot be rechecked: DeepSeek's peak rate is double off-peak from 2026-08-17,
so recomputing an old row at "the current rate" silently produces a different
answer than what was actually billed.

Migration 0002 adds both columns. tokens_reasoning defaults to 0 rather than
NULL so cost aggregates do not collapse to NULL on historical rows;
rate_tier stays nullable because rows written before this change genuinely
have an unknown tier, and NULL says that more honestly than a guessed 'flat'.

The smoke script now asserts both: reasoning tokens must be 0 (a non-zero
value means a provider default re-enabled thinking mode behind our backs) and
rate_tier must be present. Verified against the real API -- tokens_reasoning=0,
rate_tier=off_peak, 3.9s round trip.

.secrets.baseline

@@ -231,7 +231,16 @@
         "is_verified": false,
         "line_number": 49
       }
+    ],
+    "tests/unit/test_thinking_mode.py": [
+      {
+        "type": "Secret Keyword",
+        "filename": "tests/unit/test_thinking_mode.py",
+        "hashed_secret": "3acfb2c2b433c0ea7ff107e33df91b18e52f960f",
+        "is_verified": false,
+        "line_number": 48
+      }
     ]
   },
-  "generated_at": "2026-08-13T16:53:15Z"
+  "generated_at": "2026-08-13T17:02:10Z"
 }