fix(dbmanager): keep the pool alive across errors and restarts

Implements the fixes from audit/pkg/dbmanager.audit.md.

- Stop closing the shared *sql.DB to recover from errors. Adapter
  factories and the health checker no longer call Reconnect; Reconnect is
  atomic and operator-only.
- Postgres uses a custom driver.Connector: Reconnect retires pooled
  connections by generation without closing the pool, so held Bun/GORM
  handles keep working. Verified against a live server restart.
- Add TCP keepalive, TCP_USER_TIMEOUT, a bounded reuse ping and
  statement_timeout as a runtime parameter; drop the 2 min timeout floor.
- Health check pings without holding the connection lock.
- Listener: single goroutine pair, bounded Close without UNLISTEN, and
  serialised use of the pgx connection (fixes conn busy and a close race).
- Fix Connect/Close/Connect/Close panic, idempotent Connect, dial outside
  the manager lock, clean up on partial failure.
- SQLite: pin :memory: to one connection, pragmas via DSN.
- Escape credentials in Postgres/MSSQL/Mongo DSNs; sslmode defaults to
  prefer. Wire retry settings, publish metrics, fix logger calls.
- NewConnectionFromDB: Close is a no-op with a warning (caller owns the
  pool); Reconnect only pings.
- Document correct usage in the README; mark the audit with what was done.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

pkg/dbmanager/README.md

@@ -50,7 +50,6 @@ dbmanager:
 
   # Health checks
   health_check_interval: 30s
-  enable_auto_reconnect: true
 
   connections:
     # Primary PostgreSQL connection
@@ -256,7 +255,7 @@ db, _ := mgr.GetDefaultDatabase()
 | `retry_delay` | duration | 1s | Initial retry delay |
 | `retry_max_delay` | duration | 10s | Maximum retry delay |
 | `health_check_interval` | duration | 30s | Interval between health checks |
-| `enable_auto_reconnect` | bool | true | Auto-reconnect on health check failure |
+| `enable_auto_reconnect` | bool | - | Deprecated and ignored: the manager never closes the pool to recover from errors |
 
 ### Connection Configuration
 
@@ -451,7 +450,6 @@ db.NewSelect().Model(&User{}).Scan(ctx)
 3. **Enable Health Checks**: Catch connection issues early
    ```yaml
    health_check_interval: 30s
-   enable_auto_reconnect: true
    ```
 
 4. **Use Appropriate ORM**: Choose based on your needs