Implement GET /analytics/revenue-chart`

[Nemenwq]

Dependencies: Issue #27, Issue #28.

Context:
Provides chart data for the frontend.

Tasks:

• Return a normalized time series:
  • [{ date: "YYYY-MM-DD", volume: number, count: number }]
• Enforce merchant scoping.

Implementation Notes:

• Ensure date bucketing uses a consistent timezone (usually UTC).

Scope:

• In scope: Return daily series for 30 days.
• Out of scope: Custom ranges.

Files: backend/payment_service/src/analytics/