Remove the sync.Mutex and CreateUserAtomic (INSERT ON CONFLICT) in favor
of a single DB transaction in CreateFirstUser that atomically checks for
existing users and inserts. SQLite serializes write transactions, so this
is sufficient to prevent the race condition without application-level locking.
- Store lastSeen timestamp per IP limiter entry
- Lazy sweep removes entries older than 10 minutes on each request
- Add Retry-After header to 429 responses
- Add test for stale entry eviction
Fixes memory leak under sustained attack from many IPs.
user