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>

docker-compose.yml

@@ -71,6 +71,9 @@ services:
       BIND_ADDRESS: 0.0.0.0:8080
       STORAGE_DIR: /var/lib/mangalord/storage
       RUST_LOG: ${RUST_LOG:-info,mangalord=debug}
+      # Postgres connection-pool sizing — see .env.example for context.
+      DB_MAX_CONNECTIONS: ${DB_MAX_CONNECTIONS:-20}
+      DB_ACQUIRE_TIMEOUT_SECS: ${DB_ACQUIRE_TIMEOUT_SECS:-10}
       # Auth / cookies — see .env.example for context.
       COOKIE_SECURE: ${COOKIE_SECURE:-true}
       COOKIE_DOMAIN: ${COOKIE_DOMAIN:-}