chore(backend): bootstrap Python 3.12+ project skeleton (B0.1)

- pyproject.toml with ruff + mypy strict + pytest + coverage >=70%
- Makefile with Docker/Podman auto-detect
- Multi-stage Dockerfile (python:3.12-slim-bookworm, non-root user)
- docker-compose.yml for Postgres dev DB
- alembic.ini wired to src/mimic/db/migrations
- scripts/postgres-init/00-roles.sql seeds the audit writer role
- .env.example documents every MIMIC_* var (no secrets committed)

backend/docker-compose.yml

@@ -0,0 +1,23 @@
+services:
+  postgres:
+    image: postgres:16-alpine
+    container_name: mimic-postgres
+    restart: unless-stopped
+    environment:
+      POSTGRES_DB: ${POSTGRES_DB:-mimic}
+      POSTGRES_USER: ${POSTGRES_USER:-mimic_app}
+      POSTGRES_PASSWORD: ${POSTGRES_PASSWORD:-mimic_dev_password}
+    ports:
+      - "127.0.0.1:5432:5432"
+    volumes:
+      - mimic_pgdata:/var/lib/postgresql/data
+      - ./scripts/postgres-init:/docker-entrypoint-initdb.d:ro
+    healthcheck:
+      test: ["CMD-SHELL", "pg_isready -U ${POSTGRES_USER:-mimic_app} -d ${POSTGRES_DB:-mimic}"]
+      interval: 5s
+      timeout: 3s
+      retries: 10
+
+volumes:
+  mimic_pgdata:
+    name: mimic_pgdata