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/telemetry/postgres.py

@@ -48,7 +48,9 @@ CREATE TABLE IF NOT EXISTS llm_calls (
     cached_prompt_tokens INTEGER,
     model_reported       TEXT,
     sampling             TEXT,
-    reasoning_tokens     INTEGER
+    reasoning_tokens     INTEGER,
+    tenant_id            TEXT NOT NULL DEFAULT '',
+    meta                 JSONB NOT NULL DEFAULT '{}'::jsonb
 );
 """
 
@@ -58,6 +60,15 @@ _BACKFILL = (
     ("model_reported", "ALTER TABLE llm_calls ADD COLUMN model_reported TEXT"),
     ("sampling", "ALTER TABLE llm_calls ADD COLUMN sampling TEXT"),
     ("reasoning_tokens", "ALTER TABLE llm_calls ADD COLUMN reasoning_tokens INTEGER"),
+    # 两个默认值都是非易失常量,PG 11+ 只改 catalog 不重写全表,故大表补列亦是秒级
+    (
+        "tenant_id",
+        "ALTER TABLE llm_calls ADD COLUMN tenant_id TEXT NOT NULL DEFAULT ''",
+    ),
+    (
+        "meta",
+        "ALTER TABLE llm_calls ADD COLUMN meta JSONB NOT NULL DEFAULT '{}'::jsonb",
+    ),
 )
 
 # 探测表是否存在;不需要任何权限,且与 INSERT 走同一套 search_path 解析
@@ -92,6 +103,8 @@ _COLUMNS = (
     "model_reported",
     "sampling",
     "reasoning_tokens",
+    "tenant_id",
+    "meta",
 )
 
 _INSERT = (