Queue System Simplification Implementation

I have successfully implemented the queue system simplification plan.

Changes Implemented

Added claimNextMessage(sessionDbId) to PendingMessageStore:
- Implements an atomic transaction (SELECT oldest pending + UPDATE to processing).
- Ensures a message can only be claimed by one worker at a time.
- Eliminates race conditions between "peeking" and "marking".
Removed peekPending():
- No longer needed as claimNextMessage handles retrieval and locking in one step.

Created src/services/queue/SessionQueueProcessor.ts:
- Implements a robust AsyncIterableIterator that yields messages.
- Encapsulates the "Claim -> Yield -> Wait" loop.
- Replaces fragile polling/recursive logic with event-driven waitForMessage.
- Handles empty queues gracefully by waiting for signals.

Updated getMessageIterator:
- Now delegates to SessionQueueProcessor.
- Removes complex manual synchronization logic.
Removed In-Memory Queue State:
- queueObservation and queueSummarize now only write to DB and emit events.
- pendingMessages array is no longer used for logic (kept deprecated for type compatibility).
- getTotalActiveWork, hasPendingMessages, etc., now query PendingMessageStore directly (counting both 'pending' and 'processing' states).

Removed Recursive Restarts:
- Refactored startGeneratorWithProvider in SessionRoutes.ts and startSessionProcessor in WorkerService.ts.
- Removed logic that deleted sessions when queue emptied (sessions now wait for new work).
- Removed "auto-restart" logic for normal completion (only kept for crash recovery).

Reliability: Atomic DB operations prevent stuck or duplicate messages.
Simplicity: Removed complex "peek-then-mark" and recursive restart chains.
Performance: Zero-latency event notification with efficient DB queries.
Maintainability: Clear separation of concerns (Store vs Processor vs Manager).