feat(14-02): add Docker Compose files for PostgreSQL dev and production

- Create docker-compose.dev.yml with Postgres 16 for local development
- Rewrite docker-compose.yml with Postgres service, healthcheck, and app dependency chain
- Production uses externalized POSTGRES_PASSWORD and DATABASE_URL env vars

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docker-compose.dev.yml

@@ -0,0 +1,19 @@
+services:
+  postgres:
+    image: postgres:16-alpine
+    environment:
+      POSTGRES_USER: gearbox
+      POSTGRES_PASSWORD: gearbox
+      POSTGRES_DB: gearbox
+    ports:
+      - "5432:5432"
+    volumes:
+      - pgdata-dev:/var/lib/postgresql/data
+    healthcheck:
+      test: ["CMD-SHELL", "pg_isready -U gearbox"]
+      interval: 5s
+      timeout: 3s
+      retries: 5
+
+volumes:
+  pgdata-dev: