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

wwwroot/js/session.js

@@ -1,7 +1,12 @@
 // ── Session State ──
 
 let currentSessionId = null;
+let currentAgentId = null;
 
 function getCurrentSessionId() {
   return currentSessionId;
 }
+
+function getCurrentAgentId() {
+  return currentAgentId;
+}