refactor: make the telemetry schema a single source of truth

DDL, column order and backfill statements lived twice, once in each
recorder. A public telemetry_schema_sql() would have made three copies,
and the drift shows up downstream as "I ran the printed SQL and the
library still reports a missing column".

Move both DDLs, both backfill lists and the 24 INSERT fields into
telemetry/schema.py verbatim; the recorders now import them and build
_INSERT through insert_sql(backend, COLUMNS) at import time. The
generated statements are byte-identical to the previous constants, so
runtime behaviour is unchanged (the postgres conflict target stays
bound to call_id for now).

insert_sql() validates its columns against COLUMNS: from the next task
on those names come from database probing, not from a constant, so the
subset check is the gate on the only injection surface. The new
telemetry_schema_sql() prints a paste-ready migration script; its
postgres backfill deliberately uses ADD COLUMN IF NOT EXISTS while the
library's own statements do not, because that form takes an ACCESS
EXCLUSIVE lock even when the column exists. Both variants are derived
from one declaration list so their column sets cannot drift.

tests/integration/test_postgres_telemetry.py

@@ -320,7 +320,7 @@ async def least_privilege_dsn(dsn):
     """
     import asyncpg
 
-    from polygateway.telemetry.postgres import _DDL
+    from polygateway.telemetry.schema import PG_DDL
 
     name = f"pgwtest_lp_{uuid4().hex[:8]}"
     admin = await asyncpg.connect(dsn, timeout=10)
@@ -332,7 +332,7 @@ async def least_privilege_dsn(dsn):
         await admin.execute(f"CREATE ROLE {name} LOGIN PASSWORD '{_PROBE_PASSWORD}'")
         await admin.execute(f"CREATE SCHEMA {name}")
         await admin.execute(f"SET search_path = {name}")
-        await admin.execute(_DDL)  # 表由**别的账号**建好,与现场一致
+        await admin.execute(PG_DDL)  # 表由**别的账号**建好,与现场一致
         await admin.execute(f"GRANT USAGE ON SCHEMA {name} TO {name}")
         await admin.execute(f"GRANT SELECT, INSERT ON {name}.llm_calls TO {name}")
         # 关键: 绝不 GRANT CREATE ON SCHEMA —— 缺的正是这一项