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>

.gitea/workflows/deploy.yml

@@ -3,6 +3,8 @@ name: deploy
 on:
   push:
     branches: [main]
+  pull_request:
+    branches: [main]
   workflow_dispatch:
 
 jobs:
@@ -63,6 +65,10 @@ jobs:
   build-and-push:
     runs-on: ubuntu-latest
     needs: [test-backend, test-frontend]
+    # PRs only run the test jobs; build + deploy are reserved for
+    # post-merge pushes to main. Without this gate every PR would push
+    # a tagged image to the registry and SSH-deploy to prod.
+    if: github.event_name != 'pull_request'
     outputs:
       image_tag: ${{ steps.meta.outputs.image_tag }}
       version: ${{ steps.meta.outputs.version }}
@@ -117,6 +123,7 @@ jobs:
   deploy:
     runs-on: ubuntu-latest
     needs: build-and-push
+    if: github.event_name != 'pull_request'
     steps:
       - name: SSH deploy
         uses: appleboy/ssh-action@v1.0.3