Retire idle persistent MySQL connections

docs/setup.md

@@ -203,6 +203,7 @@ WASM_EPOCH_DEADLINE_TICKS=200
 WASM_EPOCH_PERIOD_MS=50
 WASM_INVOCATION_TIMEOUT_MS=30000
 MYSQL_PERSISTENT_POOL_SIZE=8
+MYSQL_PERSISTENT_POOL_IDLE_TIMEOUT_SECONDS=300
 
 WORKER_COUNT=4
 MAX_MEMORY=16777216
@@ -225,6 +226,7 @@ Important settings:
 - `TMP_UPLOAD_PATH` and `SESSION_PATH` must be writable by the runtime.
 - `SESSION_COOKIE_SECURE=1` adds the `Secure` attribute to UCE-managed session cookies and should be used for HTTPS-only deployments. Leave it `0` only for local/plain-HTTP development.
 - `MYSQL_PERSISTENT_POOL_SIZE` caps credential-keyed connections retained by each Wasm worker. The default `8` is clamped to `64`; set it to `0` to restore request-lifetime connections. Cached sessions are reset before reuse.
+- `MYSQL_PERSISTENT_POOL_IDLE_TIMEOUT_SECONDS` retires a pooled connection at the first request boundary after it has been idle for this interval (default `300`, maximum `86400`). Set it to `0` to retain idle connections until capacity eviction, reset failure, worker exit, or database-side closure. Keep a positive value below the database server's idle timeout to avoid attempting stale sessions after traffic resumes. A completely idle UCE worker has no request boundary and therefore does not proactively close sockets.
 - `HTTP_PORT` is the built-in HTTP/WebSocket listener used for WebSocket upgrade traffic and direct local probes. Bind/firewall it for local access only; nginx/Apache should be the public entry point.
 - `WS_BROKER_OUTBOUND_TIMEOUT_SECONDS` controls how long a forwarded WS message can remain queued in the broker before being dropped (default `30`). Set to `0` to disable the timeout.
 - `WASM_COMPILE_SCRIPT` must point to `scripts/compile_wasm_unit` unless you provide an equivalent compiler. Relative paths are resolved from the runtime root/`COMPILER_SYS_PATH`. That script calls `scripts/check_unit_wasm.py` after linking each unit and uses the pinned WASI SDK on every deployment host.