Add persistent chat session state (ChatState, sequence tracking, tool
calls). Introduce basic billing record in agent crate. Refine chat
service to route messages through state machine with tool support.
Critical fixes:
- Wrap balance updates in database transactions with SELECT FOR UPDATE
- Move history insert after balance validation to prevent orphaned records
- Use Decimal throughout to avoid silent conversion failures
- Prevent concurrent requests from causing negative balances
Tasks resolved:
- Task #4: Silent Decimal conversion failures
- Task #5: Missing transaction isolation (race conditions)
- Task #6: History inserted before validation
