feat: Telegram Bot Integration

- TelegramService: Non-Streaming Claude Chat mit Tool-Loop,
  Commands (/start, /link, /session, /new, /clear, /status, /unlink),
  Nachrichten-Splitting bei >4096 Zeichen, Typing-Indicator,
  History-Aufbau aus DB, Token-Usage-Tracking
- TelegramEndpoint: Webhook (POST, no-auth), Link-Code-Generierung
  (POST, auth), Webhook-Registrierung + Info (Admin)
- DatabaseService: telegram_links Tabelle mit Link-Code-Flow
  (6-stelliger Code, 5 Min Ablauf), GetUserSessions
- AuthMiddleware: /api/telegram/webhook von Auth ausgenommen
- Session-Verknüpfung via Link-Code (Web-UI generiert Code,
  Telegram /link CODE löst ein)
- TelegramLinkRecord Model

Services/DatabaseService.cs

@@ -183,6 +183,22 @@ public class DatabaseService
                 updated_at    TEXT NOT NULL
             );
         ");
+
+        conn.Execute(@"
+            CREATE TABLE IF NOT EXISTS telegram_links (
+                id               INTEGER PRIMARY KEY AUTOINCREMENT,
+                telegram_chat_id INTEGER NOT NULL,
+                user_id          TEXT NOT NULL,
+                session_id       TEXT NOT NULL,
+                agent_id         TEXT,
+                link_code        TEXT,
+                expires_at       TEXT,
+                created_at       TEXT NOT NULL,
+                updated_at       TEXT NOT NULL,
+                UNIQUE(telegram_chat_id)
+            );
+            CREATE INDEX IF NOT EXISTS idx_telegram_links_code ON telegram_links(link_code);
+        ");
     }
 
     private void TryAddColumn(SqliteConnection conn, string table, string column, string type)
@@ -742,4 +758,87 @@ public class DatabaseService
             "UPDATE users SET display_name = @displayName, persona = @persona, updated_at = @now WHERE user_id = @userId",
             new { userId, displayName, persona, now = DateTime.UtcNow.ToString("o") });
     }
+
+    // ── Telegram ──
+
+    public async Task<string> CreateTelegramLinkCode(string userId, string sessionId, string agentId)
+    {
+        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");
+
+        // 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)
+        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;
+    }
+
+    public async Task<TelegramLinkRecord> RedeemTelegramLinkCode(string code, long 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
+        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;
+    }
+
+    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)
+    {
+        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 });
+    }
+
+    public async Task<IEnumerable<SessionRecord>> GetUserSessions(string userId)
+    {
+        using var conn = CreateConnection();
+        return await conn.QueryAsync<SessionRecord>(
+            "SELECT * FROM sessions WHERE user_id = @userId AND is_deleted = 0 ORDER BY updated_at DESC",
+            new { userId });
+    }
 }