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>

.gitignore

@@ -9,5 +9,16 @@ wheels/
 # Virtual environments
 .venv
 
-# Enviroment variables 
+# Test and coverage artifacts
+.pytest_cache/
+.coverage
+coverage.xml
+htmlcov/
+
+# Environment variables
 .env
+.env.langfuse
+infra/langfuse/.env
+
+# Claude Code local overrides
+.claude/settings.local.json