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.Users.cs

@@ -187,32 +187,43 @@ public partial class DatabaseService
         await conn.ExecuteAsync("DELETE FROM users WHERE user_id = @userId", new { userId });
     }
 
-    // ── User API-Key Auswahl ──
+    // ── Agent API-Key Zuordnung ──
 
-    public async Task SetUserSelectedApiKey(string userId, string keyId)
+    public async Task SetAgentApiKey(string userId, string agentId, string keyId)
     {
         using var conn = CreateConnection();
-        await conn.ExecuteAsync(
-            "UPDATE users SET selected_api_key_id = @keyId, updated_at = @now WHERE user_id = @userId",
-            new { userId, keyId = (object)keyId ?? DBNull.Value, now = DateTime.UtcNow.ToString("o") });
+        if (string.IsNullOrEmpty(keyId))
+        {
+            await conn.ExecuteAsync(
+                "DELETE FROM agent_api_keys WHERE user_id = @userId AND agent_id = @agentId",
+                new { userId, agentId });
+        }
+        else
+        {
+            await conn.ExecuteAsync(@"
+                INSERT INTO agent_api_keys (user_id, agent_id, key_id)
+                VALUES (@userId, @agentId, @keyId)
+                ON CONFLICT(user_id, agent_id) DO UPDATE SET key_id = @keyId",
+                new { userId, agentId, keyId });
+        }
     }
 
-    public async Task<string> GetUserSelectedApiKeyId(string userId)
+    public async Task<string> GetAgentApiKeyId(string userId, string agentId)
     {
         using var conn = CreateConnection();
         return await conn.QuerySingleOrDefaultAsync<string>(
-            "SELECT selected_api_key_id FROM users WHERE user_id = @userId",
-            new { userId });
+            "SELECT key_id FROM agent_api_keys WHERE user_id = @userId AND agent_id = @agentId",
+            new { userId, agentId });
     }
 
-    public async Task<ApiKeyRecord> GetUserSelectedApiKey(string userId)
+    public async Task<ApiKeyRecord> GetAgentApiKeyRecord(string userId, string agentId)
     {
         using var conn = CreateConnection();
         return await conn.QuerySingleOrDefaultAsync<ApiKeyRecord>(@"
             SELECT k.* FROM api_keys k
-            INNER JOIN users u ON u.selected_api_key_id = k.key_id
-            WHERE u.user_id = @userId",
-            new { userId });
+            INNER JOIN agent_api_keys a ON a.key_id = k.key_id
+            WHERE a.user_id = @userId AND a.agent_id = @agentId",
+            new { userId, agentId });
     }
 
     /// <summary>