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>
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