Add category drill-down to item rankings

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

docs/statistics.md

@@ -18,7 +18,7 @@ Statistics pages live under `/stats/*` (`src/frontend/src/views/StatisticsView.t
 ## My Orders (orderees)
 
 - ✅ Participation over time (orders joined per month) and current streak (consecutive weeks)
-- ✅ Favourite items and categories, items repeated most
+- ✅ Favourite items and categories, items repeated most (drill-down: all items, by category, or one category)
 - ✅ Total and average spend per order (priced menus only)
 - ✅ Usual order day / hour
 - ✅ Most frequent organizers
@@ -30,7 +30,7 @@ Statistics pages live under `/stats/*` (`src/frontend/src/views/StatisticsView.t
 - ✅ Orders created over time; open vs. closed (closed = ordering done)
 - ✅ Participants per order (average, trend); repeat vs. new participants
 - ✅ Submission timeline: time from order creation to submission
-- ✅ Popular items and categories across own orders
+- ✅ Popular items and categories across own orders (same category drill-down)
 - ✅ Order volume: items and value per order
 - ✅ Payment status: paid / accepted-unpaid / pending
 - 🔲 Average time to payment — only the current `paid` flag is stored. Backend: add `submissions.paid_at` (and `accepted_at`), set on status updates, backfill NULL.
@@ -44,7 +44,7 @@ Statistics pages live under `/stats/*` (`src/frontend/src/views/StatisticsView.t
 - ✅ Unconfirmed accounts, pending email / user-id changes, open registration links
 - ✅ Orders per week and share of orders with ≥1 submission
 - ✅ Top order managers, most active participants
-- ✅ Most ordered items site-wide
+- ✅ Most ordered items site-wide (same category drill-down; `items` in `/api/admin/stats` is capped at 300)
 - ✅ Storage usage per data folder and orphaned files
 - 🔲 Real daily/weekly/monthly active users — only creation timestamps are recorded. Backend: `user_profiles.last_seen_at` updated (throttled) in `get_existing_user_id`, or an `activity_log` table.
 - 🔲 Email volume and failures — `email_service` does not log sends. Backend: `email_log` table (kind, recipient hash, status, error, created_at) written by `email_service`.