feat: record the reasoning verdict in telemetry

This issue surfaced only because someone ran a slow suite that is
excluded by default and had not been run for eighteen days. As a column
it becomes a query: which model stopped being observable, and when.

The emitter unwraps the enum to a plain str at the single _record exit.
asyncpg makes no promise about encoding a str subclass, and a telemetry
write that fails is downgraded to one warning — it would not crash, it
would just quietly cost the Postgres path a column. Normalising at the
emitter follows what tenant_id, meta and sampling already do.

The column is appended last in COLUMNS and in both DDLs. An existing
table can only take ALTER at the end, so putting it anywhere else
forks the physical column order between a freshly built database and a
backfilled one.

src/polygateway/ports.py

@@ -260,13 +260,16 @@ class TelemetryStatusProvider(Protocol):
 
 @runtime_checkable
 class TelemetryRecorder(Protocol):
-    """遥测后端;24 字段冻结(M1 设计 §4.4 + issue #3/#4/#11),唯一调用点是 TelemetryEmitter。
+    """遥测后端;25 字段冻结(M1 设计 §4.4 + issue #3/#4/#11/#16),唯一调用点是 TelemetryEmitter。
 
     新增参数不设默认值: 库外无第三方实现者(三项目迁移时删除了各自的同名
     Protocol),完整签名的成本为零,而少写一列会被 emitter 的降级吞成 warning。
 
     `tenant_id` 与 `meta` 到达 recorder 时**已由 emitter 归一化**——`tenant_id`
     的 `None` 已转空串,`meta` 已序列化为 JSON 字符串(空 dict 为 `'{}'`)。
+    `thinking_observation` 同理: emitter 已把 `ThinkingObservation` 取成 `.value`
+    的裸 `str`(`StrEnum` 是 `str` 子类,而 asyncpg 的参数编码对子类不保证接受,
+    遥测写失败又只降级成 warning——PG 那一路会静默少一列数据)。
     recorder 只负责落库,不做任何语义判断,与 `sampling` 列由
     `canonical_sampling_json()` 在 emitter 侧定型是同一先例。
     """
@@ -298,4 +301,5 @@ class TelemetryRecorder(Protocol):
         reasoning_tokens: int | None,
         tenant_id: str,
         meta: str,
+        thinking_observation: str,
     ) -> None: ...