fix: resolve 429 rate limit errors by implementing CoordinatorEntity pattern

[jmdevita]

Fixes #66

The integration was hitting API rate limits (20 requests/hour) due to an antipattern where each of the 4 sensors had independent SCAN_INTERVAL timers and manually called coordinator.async_request_refresh(). This resulted in 20+ API calls per hour instead of the expected 12.

Changes:

• Refactored all 4 sensor classes to inherit from CoordinatorEntity
• Removed SCAN_INTERVAL to let the coordinator control update timing
• Replaced async_update() methods with _handle_coordinator_update()
• Removed manual async_request_refresh() calls from sensors
• Fixed services.py error message: "20 per day" → "20 per hour"

Result: Exactly 12 API calls per hour (one every 5 minutes), instead of 20+ when the sensors' timers were out of sync.