feat: API Key pro Agent statt pro User

- Neue Tabelle agent_api_keys (user_id, agent_id, key_id)
- Migration: bestehende selected_api_key_id auf alle Agents verteilt
- Backend: GetAgentApiKeyRecord(userId, agentId) in Chat, Telegram, Cronjobs
- Endpoints: select/selected mit ?agentId Query-Parameter
- Frontend: currentAgentId aus Session, alle Key-Calls agent-aware
- DeleteApiKey bereinigt agent_api_keys statt users.selected_api_key_id

Services/DatabaseService.Integrations.cs

@@ -427,9 +427,9 @@ public partial class DatabaseService
         await conn.ExecuteAsync(
             "UPDATE scanned_keys SET adopted_api_key_id = NULL WHERE adopted_api_key_id = @keyId",
             new { keyId });
-        // User die diesen Key ausgewählt haben zurücksetzen
+        // Agent-Key-Zuordnungen die auf diesen Key verweisen löschen
         await conn.ExecuteAsync(
-            "UPDATE users SET selected_api_key_id = NULL WHERE selected_api_key_id = @keyId",
+            "DELETE FROM agent_api_keys WHERE key_id = @keyId",
             new { keyId });
         await conn.ExecuteAsync("DELETE FROM api_keys WHERE key_id = @keyId", new { keyId });
     }