Fix email fetch reliability for large emails with attachments

- Add 120s IMAP socket timeout (was unlimited, could hang on large emails)
- Increase Paperless upload timeout from 60s to 300s for large attachments
- Increase manual poll UI timeout from 60s to 300s
- Show error count in UI when emails fail to process
- Log warning when attachment payload is empty/corrupted

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/stiftung/tasks.py

@@ -137,7 +137,7 @@ def _upload_to_paperless(content: bytes, filename: str, destinataer=None, betref
             headers=headers,
             data=form_data,
             files=files,
-            timeout=60,
+            timeout=300,  # 5 Minuten für große Anhänge
         )
         response.raise_for_status()
         # Paperless gibt die neue Dokument-ID zurück (als Integer oder UUID-String)
@@ -196,11 +196,12 @@ def poll_destinataer_emails(self, search_all_recent_days=0):
     errors = 0
 
     try:
-        # IMAP-Verbindung aufbauen
+        # IMAP-Verbindung aufbauen (mit Socket-Timeout für große E-Mails)
+        imap_timeout = 120  # Sekunden – genug für große Anhänge
         if imap_use_ssl:
-            mail = imaplib.IMAP4_SSL(imap_host, imap_port)
+            mail = imaplib.IMAP4_SSL(imap_host, imap_port, timeout=imap_timeout)
         else:
-            mail = imaplib.IMAP4(imap_host, imap_port)
+            mail = imaplib.IMAP4(imap_host, imap_port, timeout=imap_timeout)
 
         mail.login(imap_user, imap_password)
         mail.select(imap_folder)
@@ -279,6 +280,10 @@ def poll_destinataer_emails(self, search_all_recent_days=0):
                             filename = _decode_header_value(part.get_filename() or "")
                             content = part.get_payload(decode=True)
                             if not content:
+                                logger.warning(
+                                    "Anhang '%s' hat keinen Inhalt (möglicherweise zu groß oder beschädigt) – wird übersprungen.",
+                                    filename,
+                                )
                                 continue
 
                             doc_id = _upload_to_paperless(