Fix SonoffLAN signing key: hardcode pre-computed HMAC key

The previous _compute_sign_key() function indexed into a base64 string
derived from the full SonoffLAN REGIONS dict (~243 entries). Our partial
dict only produced a 7876-char a string but needed index 7872+, so the
function must use the full dict.

Solution: pre-compute the key once from the full dict and hardcode the
resulting 32-byte ASCII key. This is deterministic — the SonoffLAN
algorithm always produces the same output regardless of when it runs.

The sonoff_ewelink driver now loads cleanly alongside all other drivers.

app/models/__init__.py

@@ -2,5 +2,6 @@
 from .user import User
 from .board import Board
 from .workflow import Workflow
+from .sonoff_device import SonoffDevice
 
 __all__ = ["User", "Board", "Workflow"]