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

Endpoints/ApiKeysEndpoint.cs

@@ -540,8 +540,16 @@ public static class ApiKeysEndpoint
             return;
         }
 
-        await db.SetUserSelectedApiKey(userId, keyId);
-        await ctx.Response.WriteAsJsonAsync(new { ok = true, keyId, name = key.Name });
+        var agentId = ctx.Request.Query["agentId"].FirstOrDefault();
+        if (string.IsNullOrEmpty(agentId))
+        {
+            ctx.Response.StatusCode = 400;
+            await ctx.Response.WriteAsJsonAsync(new { error = "agentId required" });
+            return;
+        }
+
+        await db.SetAgentApiKey(userId, agentId, keyId);
+        await ctx.Response.WriteAsJsonAsync(new { ok = true, keyId, agentId, name = key.Name });
     }
 
     private static async Task HandleGetSelected(HttpContext ctx, DatabaseService db)
@@ -549,7 +557,14 @@ public static class ApiKeysEndpoint
         var userId = ctx.GetUserId();
         if (userId == null) { ctx.Response.StatusCode = 401; return; }
 
-        var key = await db.GetUserSelectedApiKey(userId);
+        var agentId = ctx.Request.Query["agentId"].FirstOrDefault();
+        if (string.IsNullOrEmpty(agentId))
+        {
+            await ctx.Response.WriteAsJsonAsync(new { selected = false });
+            return;
+        }
+
+        var key = await db.GetAgentApiKeyRecord(userId, agentId);
         if (key == null)
         {
             await ctx.Response.WriteAsJsonAsync(new { selected = false });