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/__init__.py

@@ -35,6 +35,7 @@ def create_app(config_name: str = "default") -> Flask:
     from app.routes.workflows import workflows_bp
     from app.routes.api import api_bp
     from app.routes.admin import admin_bp
+    from app.routes.sonoff import sonoff_bp
 
     app.register_blueprint(auth_bp)
     app.register_blueprint(dashboard_bp)
@@ -42,6 +43,7 @@ def create_app(config_name: str = "default") -> Flask:
     app.register_blueprint(workflows_bp, url_prefix="/workflows")
     app.register_blueprint(api_bp, url_prefix="/api")
     app.register_blueprint(admin_bp, url_prefix="/admin")
+    app.register_blueprint(sonoff_bp)
 
     # ── user loader ───────────────────────────────────────────────────────────
     from app.models.user import User
@@ -57,9 +59,13 @@ def create_app(config_name: str = "default") -> Flask:
 
     # ── create tables & seed admin on first run ───────────────────────────────
     with app.app_context():
+        # Import all models so their tables are registered before create_all
+        from app.models import board, user, workflow  # noqa: F401
+        from app.models import sonoff_device  # noqa: F401
         db.create_all()
         _seed_admin(app)
         _add_entities_column(app)
+        _add_config_json_column(app)
         _migrate_board_types(app)
 
     return app
@@ -77,6 +83,18 @@ def _add_entities_column(app: Flask) -> None:
             db.session.rollback()  # Column already exists — safe to ignore
 
 
+def _add_config_json_column(app: Flask) -> None:
+    """Add config_json column to boards table if it doesn't exist yet."""
+    from sqlalchemy import text
+    with app.app_context():
+        try:
+            db.session.execute(text("ALTER TABLE boards ADD COLUMN config_json TEXT DEFAULT '{}'"))
+            db.session.commit()
+            app.logger.info("Added config_json column to boards table")
+        except Exception:
+            db.session.rollback()  # Column already exists — safe to ignore
+
+
 def _migrate_board_types(app: Flask) -> None:
     """Rename legacy board_type values to current driver IDs."""
     from app.models.board import Board