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>

docs/adr/0009-postgres-sqlalchemy-alembic-schema.md

@@ -58,7 +58,11 @@ Conventions:
 
 - Use SQLAlchemy async sessions in FastAPI (`AsyncSession`) and one session per
   request/job unit of work.
-- Use Alembic for all DDL. FastAPI startup opens connections and checks
+- Use Alembic for all DDL. Bootstrap the migration environment once with
+  `uv run alembic init -t async alembic`, then commit the generated
+  `alembic.ini`, `alembic/env.py`, and `alembic/versions/` directory. Retain the
+  async Alembic template and configure it to load the application's database URL
+  and SQLAlchemy model metadata. FastAPI startup opens connections and checks
   readiness; it does not create or alter tables.
 - Prefer UUID primary keys generated by the application. Avoid integer IDs that
   leak tenant size and make distributed workers harder to compose.