feat(backend): add monitor retry backoff helper (#276)

[s6pa1rta3n-lab]

Summary

Implements monitorBackoffMs(failureCount) in backend/src/utils/monitor-retry-backoff.ts for capped exponential backoff on payment stream reconnect attempts and wires it into PaymentMonitorService.

Changes

• backend/src/utils/monitor-retry-backoff.ts: Implemented monitorBackoffMs(), isValidFailureCount(), and default configuration constants.
• backend/tests/fixtures/monitor-retry-backoff.fixture.ts: Added fixture cases for standard progression, custom options, invalid input fallbacks, and validation cases.
• backend/tests/monitor-retry-backoff.test.ts: Added unit test suite covering constants, progression fixtures, custom options, invalid inputs, edge cases, monotonicity, and upper bound caps.
• backend/src/services/payment-monitor.service.ts: Imported monitorBackoffMs() and wired it into handleError with consecutive failure count tracking.

Verification

• npm test: All 208 backend unit & integration tests pass cleanly.
• npm run typecheck: TypeScript verification passes with 0 errors.
• PR touches exactly 4 paths per issue specification.

Closes #276

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