Secures telemetry health checks against information leakage and spam

[Almikefred]

Summary

Secures telemetry health checks against information leakage and spam, optimizes payment pagination to avoid full-table loads, optimizes telemetry metrics collection by reusing instruments
and preventing label cardinality explosion, and adds consistent pagination to all API list endpoints.

Changes

• src/telemetry/health_checks.rs — secure health check manager with input validation, response redaction, and 5-second caching guard
• src/payments/pagination.rs — pagination manager with 30-second count caching; enforces max page size (100); prevents unbounded queries
• src/telemetry/metrics_optimization.rs — MetricsInstruments initialized once at startup; CardinalityLimiter bounds label sets; background export pattern documented
• src/handlers/pagination.rs — PaginationQuery, PaginationHelper, and PaginatedListResponse envelope; validates page >= 1, page_size >= 1, page_size <= 100; returns 400 on invalid params

Testing

• Health checks: valid check passes, sensitive values absent from response, rapid calls return cached result within 5s window, invalid inputs rejected
• Payments pagination: correct first/subsequent pages via offset/limit, limit enforced and capped, empty result handled gracefully, count cached on repeat calls
• Metrics: instruments verified not re-created on repeated calls, cardinality limiter rejects beyond max unique values, background export spawns without error
• API pagination: default pagination returns page 1 with page_size 20, explicit params honored, over-limit page_size rejected with 400, page beyond total returns empty data with correct
  total, invalid params return 400

Closes #505
Closes #506
Closes #507
Closes #508

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