Add event processing ordering guarantees

[meshackyaro]

Description

Stellar events processed out of order causing state inconsistencies. Need strict sequence ordering.

Component

Backend

Difficulty

🔴 Hard

Tasks

• Implement sequence-based queue
• Process events in ledger order
• Handle out-of-order events
• Add sequence gap detection
• Test ordering guarantees

Acceptance Criteria

• Events processed in sequence order
• Out-of-order events buffered
• State consistency maintained

Estimated Time

3-4 days