feat: Ältere Nachrichten nachladen per Button (500er Batches)

- Backend: beforeId-Parameter + GetActiveMessagesBeforeId + GetActiveMessageCountBeforeId
- Frontend: Button 'X ältere Nachrichten laden' statt Info-Text
- Archiv-Button erscheint erst wenn alle älteren Nachrichten geladen
- Scroll-Position bleibt beim Nachladen erhalten
- Fix: wwwroot Quell-Dateien mit publish synchronisiert

Services/DatabaseService.cs

@@ -514,6 +514,28 @@ public class DatabaseService
         return messages.Reverse().ToList();
     }
 
+    public async Task<int> GetActiveMessageCountBeforeId(string sessionId, long beforeId)
+    {
+        using var conn = CreateConnection();
+        return await conn.ExecuteScalarAsync<int>(@"
+            SELECT COUNT(*) FROM messages m
+            JOIN chains c ON m.chain_id = c.chain_id
+            WHERE m.id < @beforeId AND c.archive_id IS NULL AND c.session_id = @sessionId",
+            new { sessionId, beforeId });
+    }
+
+    public async Task<List<MessageRecord>> GetActiveMessagesBeforeId(string sessionId, long beforeId, int limit)
+    {
+        using var conn = CreateConnection();
+        var messages = await conn.QueryAsync<MessageRecord>(@"
+            SELECT m.* FROM messages m
+            JOIN chains c ON m.chain_id = c.chain_id
+            WHERE m.id < @beforeId AND c.archive_id IS NULL AND c.session_id = @sessionId
+            ORDER BY m.id DESC LIMIT @limit",
+            new { sessionId, beforeId, limit });
+        return messages.Reverse().ToList();
+    }
+
     public async Task<IEnumerable<MessageRecord>> GetActiveMessagesAfterId(string sessionId, long afterId, int limit = 200)
     {
         using var conn = CreateConnection();