robust charset decoding + retry-failed endpoint

- fix: fetch_mail crashed on MIME pseudo-encodings like 'unknown-8bit',
  'x-unknown', '8bit'. New _safe_decode helper maps those (and any
  LookupError from unknown codecs) to latin-1, which never fails on
  8-bit input. Used in _extract_body and _decode_header_value.
- Consequence of the crash: scheduler marked the affected UIDs as
  processed to avoid retry loops, so those mails never got sorted
  even after the underlying issue was fixable.
- feat: POST /api/filters/retry-failed drops the ProcessedMail markers
  for UIDs that appear in "Fehler beim Abrufen" error logs, so the
  next poll re-evaluates them. Reachable via a button in the log UI
  ("Fehlgeschlagene neu einlesen"), optionally scoped per account.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app/templates/logs.html

@@ -29,6 +29,7 @@
     </label>
     <div role="group" style="margin-bottom:0;">
         <button class="outline small" onclick="loadLogs()">Suchen</button>
+        <button class="outline small" onclick="retryFailed()" title="Löscht den 'verarbeitet'-Marker für Mails, die einen Fetch-Fehler hatten, damit sie beim nächsten Poll neu versucht werden">Fehlgeschlagene neu einlesen</button>
         <button class="outline small contrast" onclick="clearLogs()">Log leeren</button>
     </div>
     <label style="margin-bottom:0;">
@@ -236,6 +237,20 @@ async function clearLogs() {
     loadLogs();
 }
 
+async function retryFailed() {
+    if (!confirm('Alle Mails mit "Fehler beim Abrufen" für den nächsten Poll neu vormerken?')) return;
+    const accountId = document.getElementById('log-account').value;
+    let url = '/api/filters/retry-failed';
+    if (accountId) url += `?account_id=${accountId}`;
+    try {
+        const resp = await fetch(url, {method: 'POST'});
+        const data = await resp.json();
+        alert(`${data.unique_uids || 0} UID(s) neu vorgemerkt (${data.reset || 0} ProcessedMail-Einträge gelöscht). Werden beim nächsten Poll neu versucht.`);
+    } catch(e) {
+        alert('Fehler: ' + e.message);
+    }
+}
+
 function toggleAutoRefresh() {
     if (document.getElementById('auto-refresh').checked) {
         refreshTimer = setInterval(() => loadLogs(currentOffset), 5000);