fix: document and wire the DB pool env vars into compose

The new DB_MAX_CONNECTIONS / DB_ACQUIRE_TIMEOUT_SECS knobs were read by the
backend but never added to .env.example or docker-compose.yml, so an operator
setting them in .env would see no effect (caught by the config meta-tests).
Document both in .env.example and wire them into backend.environment.

Bump to 0.124.8.

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

frontend/package.json

@@ -1,6 +1,6 @@
 {
   "name": "mangalord-frontend",
-  "version": "0.124.7",
+  "version": "0.124.8",
   "private": true,
   "type": "module",
   "scripts": {