Add Calculation Complexity Limits

[Marcin99b]

The API should protect itself from requests that would take too long or use too much memory.

What to do:
Add limits on calculation complexity and reject requests that would be too expensive to process.

Why this matters:

• Prevents one user from slowing down the service
• Protects against accidental or malicious overload
• Enables predictable performance
• Required for production deployment

Acceptance criteria:

• Limit number of payment items per request
• Limit calculation time period
• Limit complexity of recurring patterns
• Return helpful error when limits exceeded
• Limits are configurable