chore: run CI on PRs, require POSTGRES_PASSWORD, document HTTPS need

- .gitea/workflows/deploy.yml: trigger on pull_request to main so PRs
  get test feedback; gate build-and-push + deploy on push events so
  PRs only run the test jobs (no registry push, no SSH deploy).
- docker-compose.yml: change `${POSTGRES_PASSWORD:-mangalord}` to
  `${POSTGRES_PASSWORD:?...}` so a deploy without an .env fails fast
  instead of booting Postgres with a known-default credential.
- .env.example: change the example value to a "change-me" sentinel,
  add a banner explaining that production needs HTTPS in front of
  the frontend container because COOKIE_SECURE=true makes browsers
  refuse cookies over plain HTTP.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docker-compose.yml

@@ -1,9 +1,15 @@
+# Production-like compose. Requires a populated `.env` next to this
+# file: at minimum POSTGRES_PASSWORD must be set to a non-default
+# value (the `?required` form below fails fast otherwise). The
+# frontend container expects HTTPS in front (Caddy/Traefik/nginx)
+# because COOKIE_SECURE=true browsers will refuse to send the session
+# cookie over plain HTTP.
 services:
   postgres:
     image: postgres:16-alpine
     environment:
       POSTGRES_USER: ${POSTGRES_USER:-mangalord}
-      POSTGRES_PASSWORD: ${POSTGRES_PASSWORD:-mangalord}
+      POSTGRES_PASSWORD: ${POSTGRES_PASSWORD:?POSTGRES_PASSWORD must be set in .env}
       POSTGRES_DB: ${POSTGRES_DB:-mangalord}
     volumes:
       - postgres-data:/var/lib/postgresql/data
@@ -19,7 +25,7 @@ services:
       postgres:
         condition: service_healthy
     environment:
-      DATABASE_URL: postgres://${POSTGRES_USER:-mangalord}:${POSTGRES_PASSWORD:-mangalord}@postgres:5432/${POSTGRES_DB:-mangalord}
+      DATABASE_URL: postgres://${POSTGRES_USER:-mangalord}:${POSTGRES_PASSWORD:?POSTGRES_PASSWORD must be set in .env}@postgres:5432/${POSTGRES_DB:-mangalord}
       BIND_ADDRESS: 0.0.0.0:8080
       STORAGE_DIR: /var/lib/mangalord/storage
       RUST_LOG: ${RUST_LOG:-info,mangalord=debug}