[SUBSCRIBER] Build message validation logic

[ezehf2]

Extend subscriber.py to validate every incoming message before accepting it. Validation must check that the message is valid JSON, that all required fields are present (room, sensor_type, value, unit, timestamp), and that the value field is a number. Any message that fails validation must be rejected and logged with a clear error message explaining which field failed. Valid messages should be passed to the database insert function once Timi's database.py is ready. Commit with message [SUBSCRIBER] Add message validation logic and reference this issue.