Implement GET /analytics/overview for merchant dashboard`

[Nemenwq]

Dependencies: Issue #27, Issue #28.

Context:
Dashboard overview needs aggregated metrics.

Tasks:

• Define the metrics needed (example):
  • total payments (last 30d)
  • total volume
  • pending count
  • confirmed count
  • recent payments list (last N)
• Ensure merchant scoping is enforced.

Implementation Notes:

• Keep it efficient: single query per metric group where possible; add indexes if needed.

Scope:

• In scope: Compute basic aggregates.
• Out of scope: Data warehouse.

Files: backend/payment_service/src/analytics/