Add integration tests for client/server flow

[meonthewire]

Summary

No end-to-end tests exist for the client -> server -> storage flow.

No integration tests for the server

There are no end-to-end tests that exercise the server through its wire protocol. The full request lifecycle connecting, authenticating, performing operations, and receiving responses is only validated manually.

Test cases

• Auth -> PUT -> GET -> DEL -> STATS (happy path)
• Auth failure (wrong password)
• Permission denied (non-admin user performing admin operations)
• TTL expiration (SETEX, wait, verify key is gone)
• Batch operations (MGET/MSET/MDEL)
• List operations (LPUSH/RPUSH/LPOP/RPOP/LRANGE)
• Set operations (SADD/SREM/SMEMBERS)
• Counter operations (INCR/DECR)
• Graceful shutdown (verify clean exit)

Acceptance Criteria

• All integration tests pass in CI
• Tests are self-contained
• Each test uses a fresh server instance