feat(P4-004): summarization pipeline — LLM + cron scheduling

Add SummarizationService that reads hot agent logs (>24h), groups by
session, calls a cheap LLM (gpt-4o-mini default, configurable via
SUMMARIZATION_MODEL) to extract key insights, stores them with
embeddings in the insights table, and transitions processed logs to
warm tier. Add CronService with node-cron for scheduled execution
(summarization every 6h, tier management daily at 3am). Tier
management promotes warm→cold (30d) and purges cold logs (90d).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/gateway/package.json

@@ -36,6 +36,7 @@
     "@sinclair/typebox": "^0.34.48",
     "better-auth": "^1.5.5",
     "fastify": "^5.0.0",
+    "node-cron": "^4.2.1",
     "reflect-metadata": "^0.2.0",
     "rxjs": "^7.8.0",
     "socket.io": "^4.8.0",
@@ -43,6 +44,7 @@
   },
   "devDependencies": {
     "@types/node": "^22.0.0",
+    "@types/node-cron": "^3.0.11",
     "@types/uuid": "^10.0.0",
     "tsx": "^4.0.0",
     "typescript": "^5.8.0",