chore: add application package scaffold and tooling ignores

Add the initial src/ package (empty FastAPI entrypoint, Settings stub),
an .env.example placeholder, and expand .gitignore for test/coverage
artifacts, environment files, and local Claude Code overrides. Also
documents the Alembic async bootstrap command in ADR-0009.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/config.py

@@ -0,0 +1,5 @@
+from pydantic_settings import BaseSettings
+
+
+class Settings(BaseSettings):
+    pass