fix(P2): add OperationalError retry to sql_insert, sql_select, sql_insert_or_update

All three were missing the transient-connection retry that sql_update and
run_sql_select already had. On OperationalError (stale/dropped connection),
each now retries once with a fresh engine.connect() without disposing the pool.

IntegrityError (duplicate key, FK violation, NOT NULL) continues to return
None without retrying — the same data would fail again and None signals a
data conflict to callers, distinct from False (error) or an int (success).

sql_insert_or_update retry is safe because ON DUPLICATE KEY UPDATE is idempotent.
sql_insert retry is safe because OperationalError means MariaDB rolled back.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

documentation/TODO__Agents.md

@@ -17,7 +17,7 @@
 
 - [x] **[P1] Remove zombie `db_connection.py` import** — `app/routers/api.py` imports `db` from `app/db_connection.py`, creating a parasitic second SQLAlchemy engine at startup that is never updated by `reconnect_db()` after bootstrap. The imported `db` is only used in a commented-out line (`api.py:268`). Fix: remove the import; delete or archive `db_connection.py`.
 - [x] **[P1] Fix retry mechanism in `sql_update` / `run_sql_select`** — On `OperationalError`, both call `sql_connect()` → `reconnect_db()` which calls `engine.dispose()`, nuking the entire connection pool mid-flight. Under concurrent requests this kills other in-flight connections. Fix: remove the `sql_connect()` retry call; SQLAlchemy's `pool_pre_ping=True` already handles stale connections — just open a fresh `engine.connect()` for the retry without disposing the pool.
-- [ ] **[P2] Add retry logic to `sql_insert` and `sql_select`** — Both are missing the `OperationalError` retry that `sql_update` and `run_sql_select` have. A DB blip during an INSERT (scan record, badge log, etc.) fails silently and returns `False` with no recovery attempt.
+- [x] **[P2] Add retry logic to `sql_insert` and `sql_select`** — Added `OperationalError` retry (single fresh connection attempt) to `sql_insert`, `sql_select`, and `sql_insert_or_update`. `IntegrityError` (duplicate key, FK violation) correctly bypasses retry and returns `None` — retrying the same data would fail again.
 - [x] **[P3] Guard `db = engine.connect()` in `lib_sql_core.py` with try/except** — Wrapped in try/except; sets `db = None` on failure so Docker startup race no longer crashes the worker.
     - [ ] **[P3 full]** Migrate `lib_schema_v3.py:39` and `lib_api_crud_v3.py:166` off the global `db` to `engine.connect()` context managers, then remove the global `db` entirely.
 - [x] **[P4] Expose `pool_size` / `max_overflow` as env vars** — `create_ae_engine()` calls `settings.DB.get('pool_size', 10)` but `settings.DB` property doesn't include those keys, so they're always hardcoded 10/20. Add `AE_DB_POOL_SIZE` / `AE_DB_POOL_MAX_OVERFLOW` to `config.py`.