refactor: remove legacy SearXNG API references and web search settings

- Deleted the migration script for adding web search columns to the database.
- Removed web search related fields from the SearchSpace model and schemas.
- Eliminated web search health check endpoint and associated service logic.
- Updated frontend components to remove web search settings management.
- Cleaned up references to SearXNG API in various modules, transitioning to a platform service approach.

surfsense_backend/app/db.py

@@ -1204,12 +1204,6 @@ class SearchSpace(BaseModel, TimestampMixin):
         Integer, nullable=True, default=0
     )  # For image generation, defaults to Auto mode
 
-    # Platform web search capability (opt-out via toggle)
-    web_search_enabled = Column(
-        Boolean, nullable=False, default=True, server_default=text("true")
-    )
-    web_search_config = Column(JSONB, nullable=True)
-
     user_id = Column(
         UUID(as_uuid=True), ForeignKey("user.id", ondelete="CASCADE"), nullable=False
     )