fix: align heavy-runtime env vars with Rust RUNTIME_ prefix

- MAX_CONCURRENCY → RUNTIME_MAX_CONCURRENCY
- POLL_INTERVAL_MS → RUNTIME_POLL_INTERVAL_MS
- JOB_TIMEOUT_SECONDS → RUNTIME_JOB_TIMEOUT_SECONDS

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

docker-compose.yml

@@ -180,9 +180,9 @@ services:
       API_INTERNAL_BASE_URL: http://api:3000
       RUNTIME_SERVICE_TOKEN: ${INTERNAL_API_KEY:-change_me_runtime_key}
       DEEPSEEK_API_KEY: ${DEEPSEEK_API_KEY:-}
-      MAX_CONCURRENCY: '4'
-      POLL_INTERVAL_MS: '5000'
-      JOB_TIMEOUT_SECONDS: '120'
+      RUNTIME_MAX_CONCURRENCY: '4'
+      RUNTIME_POLL_INTERVAL_MS: '5000'
+      RUNTIME_JOB_TIMEOUT_SECONDS: '120'
       HTTP_TIMEOUT_SECONDS: '30'
     depends_on:
       - api