Bugfix: add better limits on payments & credit mutations

[lodewiges]

Somebody tried to uplift their saldo with 100,000 euro and this made the request to Mollie fail.

In the past there have been situations where an invoice for €0 was generated which resulted in the same error, and there are situations when the treasurer needs to make credit mutations bigger than €1,000.

All these situations will be addressed with this PR by:

• Adding validation to prevent user payments above €1,000 (configurable via MAX_PAYMENT_AMOUNT)
• Adding minimum €1 validation for invoice payments (configurable via MIN_INVOICE_AMOUNT)
• Increasing the credit mutation limit from €1,000 to €5,000 to allow for larger credit mutations
• Extracting payment limits to application config for easier management
• Adding comprehensive test coverage for all boundary values
• Improving validation error messages for clarity

Summary by CodeRabbit

• Bug Fixes

  • Fixed issue where excessively large payment amounts could be submitted, preventing Mollie API failures
  • Added proper validation limits to prevent abuse while allowing legitimate large transactions

• Improvements

  • Maximum credit mutation amount increased from €1,000 to €5,000
  • New configurable payment limits (max and min) to control standard vs. invoice-based payment rules
  • User payments now capped at €1,000 (configurable)
  • Invoice payments now enforce a minimum of €1
  • All payment limits now use centralized config values with safety guards

• Tests

  • Added boundary value test coverage for payment validations
  • Updated credit mutation specs to reflect new €5,000 limit

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.72%. Comparing base (b6a0b0e) to head (040472b).

Additional details and impacted files

@@             Coverage Diff             @@
##           staging    #1230      +/-   ##
===========================================
+ Coverage    77.61%   77.72%   +0.11%     
===========================================
  Files           54       54              
  Lines         1407     1414       +7     
===========================================
+ Hits          1092     1099       +7     
  Misses         315      315              

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