chore: change local development port from 8000 to 8100

- Update docker-compose.yml to map host port 8100 -> container 8000
- Update all Playwright test BASE_URL to port 8100
- Update .env.example with documentation about port mapping
- This avoids conflicts with other services on port 8000

.env.example

@@ -1,5 +1,7 @@
 APP_HOST=0.0.0.0
 APP_PORT=8000
+# For local development, docker-compose maps host port 8100 -> container port 8000
+# This avoids conflicts with other services on port 8000
 REDIS_URL=redis://localhost:6379
 CONFIG_PATH=/app/config/settings.yaml
 TURNSTILE_SITE_KEY=1x00000000000000000000AA