feat: add Slack OAuth integration and connector routes

- Introduced Slack OAuth support with new environment variables for client ID, client secret, and redirect URI.
- Implemented Slack connector routes for OAuth flow, including authorization and callback handling.
- Updated configuration to support both new OAuth format and legacy token handling.
- Enhanced the Slack indexer to decrypt tokens when necessary, ensuring compatibility with existing encrypted credentials.
- Removed outdated Slack connector UI components and adjusted frontend logic to reflect the new integration.

surfsense_backend/app/utils/validators.py

@@ -513,7 +513,22 @@ def validate_connector_config(
             ],
             "validators": {},
         },
-        "SLACK_CONNECTOR": {"required": ["SLACK_BOT_TOKEN"], "validators": {}},
+        # "SLACK_CONNECTOR": {
+        #     "required": [],  # OAuth uses bot_token (encrypted), legacy uses SLACK_BOT_TOKEN
+        #     "optional": [
+        #         "bot_token",
+        #         "SLACK_BOT_TOKEN",
+        #         "bot_user_id",
+        #         "team_id",
+        #         "team_name",
+        #         "token_type",
+        #         "expires_in",
+        #         "expires_at",
+        #         "scope",
+        #         "_token_encrypted",
+        #     ],
+        #     "validators": {},
+        # },
         "GITHUB_CONNECTOR": {
             "required": ["GITHUB_PAT", "repo_full_names"],
             "validators": {