auto-assign filter priority (max+10) on create

New filter rules no longer default to priority 100. Server picks
max(priority)+10 per account when client sends priority=None (or 10
if the account has no rules yet), and the UI pre-fills the input
with the next value instead of a fixed 100. Avoids ambiguous ordering
when multiple rules end up with the same priority.

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

app/templates/filters.html

@@ -29,8 +29,8 @@
             </label>
             <div class="grid">
                 <label>
-                    Priorität
-                    <input type="number" id="filter-priority" value="100">
+                    Priorität <small style="opacity:0.7;">(niedriger = früher)</small>
+                    <input type="number" id="filter-priority" placeholder="auto">
                 </label>
                 <label>
                     Quellordner
@@ -179,6 +179,8 @@ const ACTION_TYPES = [
     {value: 'mark_read', label: 'Als gelesen markieren', needsParam: false, needsFolder: false},
 ];
 
+let currentFilters = [];
+
 async function loadFilters() {
     const accountId = document.getElementById('account-select').value;
     const container = document.getElementById('filters-container');
@@ -188,6 +190,7 @@ async function loadFilters() {
     }
     const resp = await fetch(`/api/filters/account/${accountId}`);
     const filters = await resp.json();
+    currentFilters = filters;
     if (filters.length === 0) {
         container.innerHTML = `
             <p>Keine Filterregeln für dieses Konto.</p>
@@ -369,7 +372,8 @@ function openNewFilter() {
     document.getElementById('dialog-title').textContent = 'Neue Filterregel';
     document.getElementById('filter-id').value = '';
     document.getElementById('filter-name').value = '';
-    document.getElementById('filter-priority').value = '100';
+    const maxPrio = currentFilters.reduce((m, f) => Math.max(m, f.priority), -Infinity);
+    document.getElementById('filter-priority').value = isFinite(maxPrio) ? (maxPrio + 10) : 10;
     document.getElementById('filter-source-folder').value = 'INBOX';
     document.getElementById('filter-stop-processing').checked = false;
     document.getElementById('conditions-list').innerHTML = '';
@@ -432,10 +436,12 @@ function collectFormData() {
             parameter: input ? (input.value || null) : null,
         });
     }
+    const prioRaw = document.getElementById('filter-priority').value.trim();
+    const prio = prioRaw === '' ? null : parseInt(prioRaw);
     return {
         account_id: parseInt(accountId),
         name: document.getElementById('filter-name').value,
-        priority: parseInt(document.getElementById('filter-priority').value),
+        priority: prio,
         source_folder: document.getElementById('filter-source-folder').value,
         stop_processing: document.getElementById('filter-stop-processing').checked,
         enabled: true,