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

tests/test_hedge_builder_playwright.py

@@ -5,7 +5,7 @@ from uuid import uuid4
 
 from playwright.sync_api import expect, sync_playwright
 
-BASE_URL = "http://127.0.0.1:8000"
+BASE_URL = "http://127.0.0.1:8100"
 ARTIFACTS = Path("tests/artifacts")
 ARTIFACTS.mkdir(parents=True, exist_ok=True)