feat: Nachrichten als wichtig markieren (Stern-Button)

- MessageRecord: neues Property IsMarked
- DatabaseService: Migration is_marked, ToggleMessageMark(), GetMarkedMessages()
- HistoryEndpoint: PUT /api/messages/{id}/mark (Toggle)
- SystemPromptBuilder: markierte Nachrichten als Referenz im System-Prompt
- ChatEndpoint + SessionEndpoint: markierte Nachrichten laden und an Builder übergeben
- Frontend: Stern-Button (☆/★) auf jeder Nachricht, Toggle per API
- CSS: .marked-Klasse mit gelbem Seitenrand, Button-Styling

Services/DatabaseService.cs

@@ -128,6 +128,7 @@ public class DatabaseService
         TryAddColumn(conn, "messages", "attachment_names", "TEXT");
         TryAddColumn(conn, "agents", "tool_choice", "TEXT DEFAULT 'any'");
         TryAddColumn(conn, "agents", "custom_commands", "TEXT");
+        TryAddColumn(conn, "messages", "is_marked", "INTEGER NOT NULL DEFAULT 0");
     }
 
     private void TryAddColumn(SqliteConnection conn, string table, string column, string type)
@@ -494,4 +495,28 @@ public class DatabaseService
             ORDER BY m.id LIMIT @limit",
             new { sessionId, afterId, limit });
     }
+
+    // ── Message Marking ──
+
+    public async Task<bool> ToggleMessageMark(long messageId)
+    {
+        using var conn = CreateConnection();
+        await conn.ExecuteAsync(
+            "UPDATE messages SET is_marked = CASE WHEN is_marked = 1 THEN 0 ELSE 1 END WHERE id = @messageId",
+            new { messageId });
+        return await conn.ExecuteScalarAsync<bool>(
+            "SELECT is_marked FROM messages WHERE id = @messageId",
+            new { messageId });
+    }
+
+    public async Task<IEnumerable<MessageRecord>> GetMarkedMessages(string sessionId)
+    {
+        using var conn = CreateConnection();
+        return await conn.QueryAsync<MessageRecord>(@"
+            SELECT m.* FROM messages m
+            JOIN chains c ON m.chain_id = c.chain_id
+            WHERE c.session_id = @sessionId AND m.is_marked = 1
+            ORDER BY m.id",
+            new { sessionId });
+    }
 }