Telegram: Bot=Session statt User-Level, Chat-Code-Refactoring

- Telegram-Config (Bot-Token, Webhook-Secret, ChatId) von User-Settings
  in sessions-Tabelle verschoben → jede Session hat ihren eigenen Bot
- Link-Code-System komplett entfernt (TelegramLinkRecord gelöscht)
- /start verknüpft automatisch, kein /link mehr nötig
- Alle Endpoints session-basiert: /api/sessions/{id}/telegram/*
- Webhook-URL pro Session: /api/telegram/webhook/{sessionId}

Refactoring: Doppelten Chat-Code eliminiert
- Neuer ChatService: PrepareContext, BuildHistoryFromDb, ResolveModelId
- Neues ChatContext-Model als gemeinsame Datenstruktur
- AnthropicService.ChatNonStreaming() für Telegram (statt eigenem Loop)
- TelegramService um ~180 Zeilen reduziert, kein eigener Claude-Loop mehr
- ChatEndpoint nutzt ChatService.PrepareContext() statt eigener Setup-Logik
- Model-Mapping nur noch an einer Stelle (3x → 1x)

Frontend
- Telegram-Section von Settings-Page in Session-Bearbeitungsseite verschoben
- Webhook-Secret readonly, wird einmalig generiert und persistent gespeichert
- Status wird automatisch geladen, Register/Delete je nach Webhook-Status
- Toast-Notification-System (showToast) in ui.js
- Link-Code-Modal und Telegram-Menüpunkt im Dashboard entfernt
- Telegram-Badge auf Session-Cards (konfiguriert/verbunden)

Bugfix: Bearer statt x-api-key für OAuth-Tokens im TelegramService

Services/DatabaseService.cs

@@ -159,6 +159,9 @@ public class DatabaseService
 
         // User-Zuordnung
         TryAddColumn(conn, "sessions", "user_id", "TEXT");
+        TryAddColumn(conn, "sessions", "telegram_bot_token", "TEXT");
+        TryAddColumn(conn, "sessions", "telegram_webhook_secret", "TEXT");
+        TryAddColumn(conn, "sessions", "telegram_chat_id", "TEXT");
         TryAddColumn(conn, "agents", "user_id", "TEXT");
         TryAddColumn(conn, "users", "is_admin", "INTEGER NOT NULL DEFAULT 0");
         TryAddColumn(conn, "users", "persona", "TEXT");
@@ -331,7 +334,9 @@ public class DatabaseService
                 s.CreatedAt,
                 s.UpdatedAt,
                 MessageCount = activeCount,
-                TotalMessageCount = totalCount
+                TotalMessageCount = totalCount,
+                TelegramConfigured = !string.IsNullOrEmpty(s.TelegramBotToken),
+                TelegramLinked = !string.IsNullOrEmpty(s.TelegramChatId)
             });
         }
         return results;
@@ -809,79 +814,45 @@ public class DatabaseService
             new { userId, settings, now = DateTime.UtcNow.ToString("o") });
     }
 
-    // ── Telegram ──
+    // ── Telegram (Session-basiert) ──
 
-    public async Task<string> CreateTelegramLinkCode(string userId, string sessionId, string agentId)
+    public async Task<SessionRecord> GetSessionByTelegramChatId(long chatId)
     {
         using var conn = CreateConnection();
-        var code = Guid.NewGuid().ToString("N")[..6].ToUpperInvariant();
-        var now = DateTime.UtcNow.ToString("o");
-        var expires = DateTime.UtcNow.AddMinutes(5).ToString("o");
+        return await conn.QuerySingleOrDefaultAsync<SessionRecord>(
+            "SELECT * FROM sessions WHERE telegram_chat_id = @chatId AND is_deleted = 0",
+            new { chatId = chatId.ToString() });
+    }
 
-        // Alten Code für diese Session löschen
-        await conn.ExecuteAsync(
-            "DELETE FROM telegram_links WHERE link_code IS NOT NULL AND user_id = @userId AND session_id = @sessionId",
-            new { userId, sessionId });
-
-        // Neuen Pending-Link-Code erstellen (noch ohne telegram_chat_id)
+    public async Task UpdateSessionTelegram(string sessionId, string botToken, string webhookSecret, string chatId)
+    {
+        using var conn = CreateConnection();
         await conn.ExecuteAsync(@"
-            INSERT INTO telegram_links (telegram_chat_id, user_id, session_id, agent_id, link_code, expires_at, created_at, updated_at)
-            VALUES (0, @userId, @sessionId, @agentId, @code, @expires, @now, @now)",
-            new { userId, sessionId, agentId, code, expires, now });
-
-        return code;
+            UPDATE sessions SET telegram_bot_token = @botToken, telegram_webhook_secret = @webhookSecret,
+                telegram_chat_id = @chatId, updated_at = @now
+            WHERE session_id = @sessionId",
+            new { sessionId, botToken, webhookSecret, chatId, now = DateTime.UtcNow.ToString("o") });
     }
 
-    public async Task<TelegramLinkRecord> RedeemTelegramLinkCode(string code, long chatId)
+    public async Task SetSessionTelegramChatId(string sessionId, string chatId)
     {
         using var conn = CreateConnection();
-        var now = DateTime.UtcNow.ToString("o");
-
-        var link = await conn.QuerySingleOrDefaultAsync<TelegramLinkRecord>(
-            "SELECT * FROM telegram_links WHERE link_code = @code AND telegram_chat_id = 0",
-            new { code });
-
-        if (link == null) return null;
-        if (string.Compare(link.ExpiresAt, now, StringComparison.Ordinal) < 0) return null;
-
-        // Bestehende Verknüpfung für diese chatId entfernen
+        // Erst alte Verknüpfung mit dieser chatId entfernen (ein Telegram-Chat = eine Session)
         await conn.ExecuteAsync(
-            "DELETE FROM telegram_links WHERE telegram_chat_id = @chatId",
-            new { chatId });
-
-        // Pending-Eintrag aktivieren
-        await conn.ExecuteAsync(@"
-            UPDATE telegram_links SET telegram_chat_id = @chatId, link_code = NULL, expires_at = NULL, updated_at = @now
-            WHERE id = @id",
-            new { chatId, now, id = link.Id });
-
-        link.TelegramChatId = chatId;
-        link.LinkCode = null;
-        return link;
+            "UPDATE sessions SET telegram_chat_id = NULL, updated_at = @now WHERE telegram_chat_id = @chatId",
+            new { chatId, now = DateTime.UtcNow.ToString("o") });
+        // Dann diese Session verknüpfen
+        await conn.ExecuteAsync(
+            "UPDATE sessions SET telegram_chat_id = @chatId, updated_at = @now WHERE session_id = @sessionId",
+            new { sessionId, chatId, now = DateTime.UtcNow.ToString("o") });
     }
 
-    public async Task<TelegramLinkRecord> GetTelegramLink(long chatId)
-    {
-        using var conn = CreateConnection();
-        return await conn.QuerySingleOrDefaultAsync<TelegramLinkRecord>(
-            "SELECT * FROM telegram_links WHERE telegram_chat_id = @chatId",
-            new { chatId });
-    }
-
-    public async Task UpdateTelegramSession(long chatId, string sessionId, string agentId)
+    public async Task ClearSessionTelegramChatId(string sessionId)
     {
         using var conn = CreateConnection();
         await conn.ExecuteAsync(
-            "UPDATE telegram_links SET session_id = @sessionId, agent_id = @agentId, updated_at = @now WHERE telegram_chat_id = @chatId",
-            new { chatId, sessionId, agentId, now = DateTime.UtcNow.ToString("o") });
-    }
-
-    public async Task DeleteTelegramLink(long chatId)
-    {
-        using var conn = CreateConnection();
-        await conn.ExecuteAsync(
-            "DELETE FROM telegram_links WHERE telegram_chat_id = @chatId",
-            new { chatId });
+            "UPDATE sessions SET telegram_chat_id = NULL, updated_at = @now WHERE session_id = @sessionId",
+            new { sessionId, now = DateTime.UtcNow.ToString("o") });
     }
 
     public async Task<IEnumerable<SessionRecord>> GetUserSessions(string userId)