fix: use read-only lock for event callbacks

[0xalpharush]

No description provided.

[dguido]

LGTM - This is a correct and valuable fix.

Assessment:

• ✅ Legitimate performance improvement using RWMutex for read-heavy event publishing
• ✅ Maintains synchronization guarantees - no new race conditions
• ✅ Removes serialization bottleneck in parallel worker event publishing
• ✅ Matches existing RWMutex pattern used elsewhere in codebase (e.g., non_persistent_cache.go)

This allows all workers to publish events concurrently rather than waiting in line.