feat: add init service to dev compose, remove redundant build sections

- Add init service behind --profile init with cc-api-dev:latest image
- Remove build sections from backend-dev and backend-test (pre-built images)
- Add env var comments referencing .env.example
- Standardize INIT_MODE=infra default, RUN_INIT=true for init service

docker-compose.yml

@@ -1,4 +1,9 @@
 services:
+  # ── Required environment variables (see .env.example) ───────────────────────
+  # APP_BOLT_URL, USER_NEO4J, PASSWORD_NEO4J — Neo4j connection
+  # SUPABASE_URL, SERVICE_ROLE_KEY — Supabase project
+  # REDIS_HOST, REDIS_PORT, REDIS_PASSWORD — Redis (optional auth)
+  # FASTAPI_SECRET_KEY, ADMIN_EMAIL — API config
   redis:
     image: redis:7-alpine
     container_name: classroomcopilot-redis