Implement transaction monitoring and anomaly detection`

[Nemenwq]

Dependencies: Issue #27, Issue #30, Issue #45.

Context:
Basic fraud/anomaly rules help reduce abuse.

Tasks:

• Implement a minimal rules engine (config-driven):
  • too many payment creations per minute per merchant
  • unusually high amount compared to merchant baseline
  • repeated failed confirmations
• Persist flags and expose a simple internal endpoint or logs.

Implementation Notes:

• Keep it explainable: when you flag, record which rule triggered and why.

Scope:

• In scope: Rules engine + flagging.
• Out of scope: ML-based detection.

Files: backend/payment_service/src/monitoring/