fix: size the DB pool and fail fast on saturation

The single Postgres pool (shared by HTTP handlers and the crawler/analysis
daemons) hardcoded max_connections=10 with no acquire_timeout, so under
saturation callers hung on the driver's silent 30s default. Add a DbConfig
(DB_MAX_CONNECTIONS default 20, DB_ACQUIRE_TIMEOUT_SECS default 10, clamped
to >=1) and apply both in PgPoolOptions.

Bump to 0.124.4.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/package.json

@@ -1,6 +1,6 @@
 {
   "name": "mangalord-frontend",
-  "version": "0.124.3",
+  "version": "0.124.4",
   "private": true,
   "type": "module",
   "scripts": {