infra: single postgres instance (casdoor DB merged) + Traefik v3 edge

- remove the separate casdoor-postgres container; casdoor uses the main
  postgres (db casdoor, created by sql/02-casdoor-db.sql on fresh volumes);
  data migrated via pg_dump/pg_restore (backups kept outside the repo)
- Traefik v3.6 as the public edge (ports 80/443): docker provider pinned to
  the dedicated traefik_network (fixed-name network), host rules for
  app/api/auth/s3/storage (env-overridable, localhost defaults), frontend
  nginx demoted to internal SPA server; docker-compose.prod.yml adds real
  domains + Let's Encrypt + HTTP→HTTPS redirect (redirect middleware in
  infra/traefik-dynamic.yml)
- v3.6 required (Traefik <=3.3 fails against Docker Engine 29.x); socket
  mounted rw (Docker Desktop rejects :ro)
- start.ps1: app URL now http://localhost, casdoor + db notes

start.ps1

@@ -25,8 +25,9 @@ Start-Sleep -Seconds 5
 docker compose ps
 
 Write-Host ""
-Write-Host "Frontend : http://localhost:3000" -ForegroundColor Green
+Write-Host "Frontend : http://localhost  (via Traefik; old :3000 is gone)" -ForegroundColor Green
 Write-Host "API docs : http://localhost:8000/docs" -ForegroundColor Green
-Write-Host "DB       : localhost:5432  (procurement / postgres / postgres)" -ForegroundColor Green
+Write-Host "Casdoor  : http://localhost:8080  (admin / 123)" -ForegroundColor Green
+Write-Host "DB       : localhost:5433  (procurement / postgres / postgres; casdoor DB inside the same instance)" -ForegroundColor Green
 Write-Host ""
 Write-Host "Stop: docker compose down   |   Full reset: docker compose down -v && up -d --build" -ForegroundColor DarkGray