Sergei bbb60a9e3f Extract magic numbers to centralized constants module
Created backend/src/config/constants.js to centralize all magic numbers
and configuration values used throughout the backend codebase.

Changes:
- Created constants.js with organized sections for:
  - SECURITY: JWT, rate limiting, password reset
  - AUTH: OTP configuration and rate limiting
  - SERVER: Port, body limits, startup delays
  - MQTT: Connection settings, cache limits
  - NOTIFICATIONS: Push settings, quiet hours, batching
  - SERIAL: Validation patterns and constraints
  - EMAIL: Template settings and defaults
  - CRON: Schedule configurations
  - STORAGE: Avatar storage settings

- Updated files to use constants:
  - index.js: JWT validation, rate limits, startup delays
  - routes/auth.js: OTP generation, rate limits, JWT expiry
  - services/mqtt.js: Connection timeouts, cache size
  - services/notifications.js: Batch size, TTL, quiet hours
  - utils/serialValidation.js: Serial number constraints

- Added comprehensive test suite (30 tests) for constants module
  - All tests passing (93 total including existing tests)
  - Validates reasonable values and consistency between related constants

Benefits:
- Single source of truth for configuration values
- Easier to maintain and update settings
- Better documentation of what each value represents
- Improved code readability by removing hardcoded numbers
- Testable configuration values

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>
2026-01-29 11:52:47 -08:00
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