feat(content): eager invalidation and optimistic lock for cqrs

M4: ADR-038 Eager Invalidation + ADR-039 Optimistic Lock

- EagerInvalidationService: Redis DEL after MySQL commit

- Cache key conventions for textbook/chapter/kp/question

- Version header check (If-Match/X-Expected-Version) on write endpoints

- 409 Conflict on version mismatch

- All write endpoints return updatedAt timestamp

services/content/package.json

@@ -31,6 +31,7 @@
     "dataloader": "^2.2.2",
     "drizzle-orm": "^0.31.0",
     "graphql": "^16.9.0",
+    "ioredis": "^5.4.0",
     "kafkajs": "^2.2.4",
     "mysql2": "^3.11.0",
     "neo4j-driver": "^5.23.0",