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/board.py

@@ -40,6 +40,9 @@ class Board(db.Model):
     firmware_version = db.Column(db.String(64), nullable=True)
     created_at = db.Column(db.DateTime, default=datetime.utcnow)
 
+    # Extra driver-specific config (JSON) – e.g. eWeLink credentials/token
+    config_json = db.Column(db.Text, default="{}")
+
     # ── relationships ────────────────────────────────────────────────
     workflows_trigger = db.relationship(
         "Workflow", foreign_keys="Workflow.trigger_board_id",
@@ -49,6 +52,10 @@ class Board(db.Model):
         "Workflow", foreign_keys="Workflow.action_board_id",
         back_populates="action_board"
     )
+    sonoff_devices = db.relationship(
+        "SonoffDevice", back_populates="board",
+        cascade="all, delete-orphan", lazy="dynamic"
+    )
 
     # ── helpers ──────────────────────────────────────────────────────
     @property
@@ -130,6 +137,21 @@ class Board(db.Model):
             "idle_label":   tdef["idle"][1],
         }
 
+    @property
+    def config(self) -> dict:
+        try:
+            return json.loads(self.config_json or "{}")
+        except (ValueError, TypeError):
+            return {}
+
+    @config.setter
+    def config(self, value: dict):
+        self.config_json = json.dumps(value)
+
+    @property
+    def is_sonoff_gateway(self) -> bool:
+        return self.board_type == "sonoff_ewelink"
+
     def get_relay_label(self, relay_num: int) -> str:
         return self.get_relay_entity(relay_num)["name"]