feat(gateway): add Slack gateway configuration

surfsense_backend/app/gateway/accounts.py

@@ -2,6 +2,8 @@
 
 from __future__ import annotations
 
+import json
+
 from sqlalchemy import select
 from sqlalchemy.ext.asyncio import AsyncSession
 
@@ -25,6 +27,19 @@ def account_token(account: ExternalChatAccount) -> str | None:
     )
 
 
+def slack_account_credentials(account: ExternalChatAccount) -> dict:
+    """Decrypt Slack gateway credentials stored as encrypted JSON."""
+    if not account.encrypted_credentials:
+        return {}
+    raw = TokenEncryption(config.SECRET_KEY or "").decrypt_token(account.encrypted_credentials)
+    try:
+        data = json.loads(raw)
+    except json.JSONDecodeError:
+        # Backward-compatible fallback if a token string was stored directly.
+        return {"bot_token": raw}
+    return data if isinstance(data, dict) else {}
+
+
 async def get_or_create_system_telegram_account(
     session: AsyncSession,
 ) -> ExternalChatAccount:
@@ -78,3 +93,18 @@ async def get_or_create_system_whatsapp_account(
     await session.flush()
     return account
 
+
+async def get_slack_account_by_team(
+    session: AsyncSession,
+    *,
+    team_id: str,
+) -> ExternalChatAccount | None:
+    result = await session.execute(
+        select(ExternalChatAccount).where(
+            ExternalChatAccount.platform == ExternalChatPlatform.SLACK,
+            ExternalChatAccount.is_system_account.is_(True),
+            ExternalChatAccount.cursor_state["team_id"].astext == team_id,
+        )
+    )
+    return result.scalars().first()
+