feat: phase 0 foundation — FastAPI scaffold + /healthz

Scaffold app/ package, pinned requirements.txt, multi-stage Dockerfile,
docker-compose.yml, and .env.example. Add typed pydantic-settings loader
with full env contract and a production validator that refuses the
dev-sentinel SECRET_KEY. Wire structlog with an APP_ENV-driven renderer
(console in dev, JSON in prod). Ship a minimal unauthenticated /healthz
returning {status, version, commit_sha} with commit SHA fed through a
GIT_COMMIT_SHA build arg.

Also mark Phase 0 complete in docs/ROADMAP.md.

app/__init__.py

@@ -0,0 +1,14 @@
+"""Chicken Babies R Us FastAPI application package.
+
+This module intentionally keeps only the package version constant. All
+runtime wiring lives in :mod:`app.main`.
+"""
+
+from __future__ import annotations
+
+# Semantic version of the application. Bump per release. Surfaced via
+# FastAPI's OpenAPI metadata and the /healthz endpoint so ops can verify
+# which build is live in a given environment.
+__version__: str = "0.1.0"
+
+__all__ = ["__version__"]