fix: probe for the telemetry table before creating it

PostgreSQL checks the schema CREATE privilege before the IF NOT EXISTS
existence test, so an account with only table-level INSERT was denied on
CREATE TABLE IF NOT EXISTS even though the table was right there and
writable. The denial set _failed and the whole recorder went no-op for
the process lifetime, silently: 150+ calls downstream lost their latency,
token and cost rows with nothing but one warning to show for it.

The probe is the direct fix. The larger fix is the criterion: structural
degradation now means "provably cannot write" (pool creation failed, or
the table is absent and cannot be created), not "something threw during
init" -- a probe or acquire failure just skips the row and retries on the
next call.

SQLite stays as it is on purpose. Measured: it short-circuits the
statement at parse time, so it passes even under another connection's
EXCLUSIVE lock or on a read-only file. A probe there would buy nothing;
the docstring now says so to keep symmetry-minded future edits away.

research-wiki/ARCHITECTURE.md

@@ -475,7 +475,7 @@ flowchart TB
 
 **`sampling` 列(2026-07-31,issue #4,端口 20 → 21)**: 列语义 = 「调用方采样意图 ⊎ 生效源 `extra_body`」的 canonical JSON,空则 NULL。**不含**结构化注入的 `response_format`——列名是采样参数,schema 不是,且数 KB schema 逐行落库会让审计表无谓膨胀。三个 emit 入口口径必须各自定死,否则同一列在不同行含义不同: `emit_attempt`(RetryMW 调用,**唯一**有生效源者)并上 `source.extra_body`;`emit_cache_hit` / `emit_terminal_failure`(TelemetryMW 最外层调用)无 source 可言,只记调用级——与 `model`/`source_name` 在终态行置空是同一先例,且缓存命中行无损(`sampling` 已进缓存 key,能命中即意味调用级参数与历史那次逐字相同)。三者统一读 `request.sampling` 而非 `request.overlay`(后者在 RetryMW 处已被结构化注入污染、在 TelemetryMW 处未被污染,直接用必然三行分叉)。OCR/embedding 路径因决策 G 剥离 `extra_body`,该列恒 NULL。
 
-(`cached_prompt_tokens`/`model_reported` 为 2026-07-31 issue #3 新增,端口由 18 字段扩为 20;两个后端在初始化期对已存在的旧表幂等补列——`CREATE TABLE IF NOT EXISTS` 不会给旧表加列,不补则每行写入都被逐行 warning 丢弃。补列一律**先探测缺列再 ALTER**(`ADD COLUMN IF NOT EXISTS` 即使列已存在也先取 ACCESS EXCLUSIVE 锁,而遥测内联 await,锁共享审计表会拖垮业务调用),且**失败只逐行降级、绝不置结构性失能标志**。新列在 DDL 里必须排在 `created_at` **之后**,与 `ALTER TABLE ADD COLUMN` 的追加位置一致,否则新建库与升级库的物理列序分叉)。链路: `session_id`/`parent_call_id` 由调用方传入贯穿(agent step → LLM call)。`messages` 落库前对多模态 part 先摘要(与缓存 key 共用同一摘要函数,§7.5)——Video-Tree 现状 base64 整段进 SQLite 导致 db 膨胀(`llm.py:330`),库内修复(2026-07-20,VT 迁移缺口 R12)。
+(`cached_prompt_tokens`/`model_reported` 为 2026-07-31 issue #3 新增,端口由 18 字段扩为 20;两个后端在初始化期对已存在的旧表幂等补列——`CREATE TABLE IF NOT EXISTS` 不会给旧表加列,不补则每行写入都被逐行 warning 丢弃。补列一律**先探测缺列再 ALTER**(`ADD COLUMN IF NOT EXISTS` 即使列已存在也先取 ACCESS EXCLUSIVE 锁,而遥测内联 await,锁共享审计表会拖垮业务调用),且**失败只逐行降级、绝不置结构性失能标志**。**建表同理(2026-08-07,issue #9)**: PG 对 schema 的 CREATE 权限检查早于 `IF NOT EXISTS` 的存在性判断(16.14 实测,只授表级 `SELECT, INSERT` 的角色写得进去却建不了表),故 PG 侧必须**先 `to_regclass` 探测、表在就不发 DDL**;SQLite 侧实测在解析期即短路(持排他锁/只读文件下该语句均通过),无同款风险,**有意不加探测**。由此把"结构性失能"的判据从「初始化时出过异常」收窄为「确定写不进去」——仅建池失败与"表确定不存在且建不出来"判死,探测/取连接失败只跳过本次并留待下次重试。新列在 DDL 里必须排在 `created_at` **之后**,与 `ALTER TABLE ADD COLUMN` 的追加位置一致,否则新建库与升级库的物理列序分叉)。链路: `session_id`/`parent_call_id` 由调用方传入贯穿(agent step → LLM call)。`messages` 落库前对多模态 part 先摘要(与缓存 key 共用同一摘要函数,§7.5)——Video-Tree 现状 base64 整段进 SQLite 导致 db 膨胀(`llm.py:330`),库内修复(2026-07-20,VT 迁移缺口 R12)。
 
 - 后端: `SQLiteRecorder`(默认;WAL + busy_timeout、`INSERT OR IGNORE` 幂等、`asyncio.to_thread` 桥接、初始化/写入失败全降级不冒泡)与 `PostgresRecorder`。
 - **单一 helper 铁律**: 遥测调用点收敛为一个内部函数/上下文管理器;Video-Tree 与 GovDoc 各有 4-5 处逐字复制的 `record_llm_call(15 个参数)` 是本条的直接教训。