feat: record which tier a call actually ran at

Twenty-five columns and not one of them answered "which tier was this?",
so the question the whole issue exists to settle - does a higher tier buy
anything - had no way to group its data.

The three emit entry points deliberately disagree, the way sampling
already does. A successful attempt records what the transport actually
sent: with EFFORT_FALLBACK=nearest a request for medium goes out as low,
and recomputing here would file the row under a tier that never left the
process. A failed attempt has no response to read, so it falls back to
the requested tier - which is exactly right for the tier errors that are
rejected before any HTTP happens, because the rejected tier is the
signal. Cache hits and terminal failures have no chosen source at all,
so a source-level tier is not a thing they could report.

emit_attempt now demands to be told whether the path reasons at all.
Embedding and OCR share the emitter but never send reasoning parameters;
without the flag a source that mistakenly carries ENABLE_THINKING would
hang a tier on a call that could not possibly have run at one.

The value lands as a plain str. StrEnum is a str subclass and asyncpg
promises nothing about encoding subclasses, and a telemetry write that
fails is only a warning - Postgres would just quietly lose the column.
NULL means nobody declared a tier, which is not the same statement as
'none', and the two must never be folded together.

tests/unit/test_openai_compat.py

@@ -123,6 +123,7 @@ async def _recorded_cost(result, source):
         latency_ms=1,
         response=response,
         error=None,
+        reasoning_applies=True,
     )
     return recorder.rows[0]["cost"]