feat: AI categorisation fallback, unsubscribe confidence scoring, one-line summaries

Heuristic classifier falls back to AI only when it can't determine a category;
unsubscribe confidence blends method reliability with an AI safety opinion and
surfaces it in the Unsubscribe Manager; emails get an on-demand AI one-line
summary in the detail pane. All AI calls degrade gracefully when AI is disabled
or the provider errors.

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

frontend/src/api/client.js

@@ -135,6 +135,7 @@ function folderToRequest(slug, page, pageSize) {
 
 export const EmailApi = {
   get:        (id) => api.get(`/email/${id}`).then((r) => r.data),
+  summary:    (id) => api.get(`/email/${id}/summary`).then((r) => r.data),
   markRead:   (id) => api.post(`/email/${id}/read`),
   markUnread: (id) => api.post(`/email/${id}/unread`),
   star:       (id) => api.post(`/email/${id}/star`),