feat(import): /api/import/clear-stuck endpoint + Clear stuck UI button — escape hatch for the autoretry-loop case the automatic sweep can't break

Operator hit 3 large PNGs stuck in 'processing' for 2 days 2026-05-25:
the existing recover_interrupted_tasks flips processing > 5min back to
queued + .delay(), but if the underlying file is unfixably broken (e.g.,
PIL OSError, also patched in 68cffce), the loop never terminates and the
'Scanning...' banner sticks at 0/0 forever blocking new scans.

/api/import/clear-stuck:
- Flips every task in pending/queued/processing to 'failed' with a clear
  marker error message
- Finalizes any 'running' ImportBatch that has no remaining active children
- Idempotent + non-destructive: rows survive, can be retried once the
  underlying cause is resolved

UI button 'Clear stuck...' sits next to 'Retry failed' / 'Clear completed'
with a warning-tonal alert in the confirm dialog explaining what it does
and recommending Retry failed once the cause is fixed.

Tests: clears mixed non-terminal states, untouches complete rows,
finalizes orphan batch, no-op when nothing stuck.

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

frontend/src/stores/import.js

@@ -92,6 +92,13 @@ export const useImportStore = defineStore('import', () => {
     await loadTasks(true)
   }
 
+  async function clearStuck() {
+    const body = await api.post('/api/import/clear-stuck')
+    await loadTasks(true)
+    await refreshStatus()
+    return body
+  }
+
   const hasMore = computed(() => tasksNextCursor.value !== null)
 
   return {
@@ -101,6 +108,6 @@ export const useImportStore = defineStore('import', () => {
     triggerError,
     loadSettings, patchSettings,
     refreshStatus, triggerScan,
-    loadTasks, setStatusFilter, retryFailed, clearCompleted
+    loadTasks, setStatusFilter, retryFailed, clearCompleted, clearStuck
   }
 })