feat: record each call's tenant and caller-defined dimensions

Both telemetry backends gain tenant_id and meta at the end of the
column list, and TelemetryEmitter fills them from the request. The two
halves ship together because the emitter is the only caller of
record_llm_call: adding the columns without filling them leaves every
row short of two keys, and the backends read those keys outside their
try block, so the KeyError degrades to a warning and the whole table
stops filling.

The columns are appended, never inserted. An old table can only gain
columns through ALTER, which puts them last; a new table built from the
DDL would put them wherever the DDL says. Anywhere but the end and the
two paths produce different physical column orders, while the INSERT
uses positional placeholders.

The two backends spell the default differently for different reasons.
SQLite refuses a NOT NULL column without a non-NULL constant default
outright, so the default is what makes the backfill legal at all. On
Postgres a non-volatile constant default is what keeps the ALTER from
rewriting the table, and NOT NULL DEFAULT '' is what keeps old rows out
of the black hole a NULL tenant_id falls into under an RLS policy.

Normalisation happens in the emitter, not the recorder, matching how
canonical_sampling_json already settles the sampling column: None
becomes the empty string, an empty mapping becomes the literal '{}'.
Keys are sorted so one set of dimensions serialises identically on
every row, and allow_nan=False is a second gate behind the entry
validation -- json.dumps would otherwise write a bare NaN, which JSONB
rejects, and the failed insert would be swallowed as a warning.

All three emit entry points read the request. Cache hits read it too,
rather than the replayed response: the dimensions answer who made this
call, not who made the one whose result is being replayed.

src/polygateway/ports.py

@@ -245,10 +245,15 @@ class StructuredOutputStrategy(Protocol):
 
 @runtime_checkable
 class TelemetryRecorder(Protocol):
-    """遥测后端;22 字段冻结(M1 设计 §4.4 + issue #3/#4),唯一调用点是 TelemetryEmitter。
+    """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。
 
     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 `'{}'`)。
+    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。
     """
 
     async def record_llm_call(
@@ -276,4 +281,6 @@ class TelemetryRecorder(Protocol):
         model_reported: str | None,
         sampling: str | None,
         reasoning_tokens: int | None,
+        tenant_id: str,
+        meta: str,
     ) -> None: ...