Add trip checklists with rule-based packing advice

Each trip gets a checklist whose items group under free-text categories
(Documents, Clothing, Toiletries, Health, Electronics, Extras first, then
any custom ones alphabetically). Items are either shared — every member
sees and can tick them, and checked_by records who — or personal to one
member, which nobody else can see or touch. Items carry an optional
quantity, drag-reorder within their category, and "Uncheck all" resets the
list for the trip home.

The "Suggestions" modal is deterministic, offline advice derived from the
trip itself (src/server/util/packing.js) — no LLM and no external calls, so
it stays unit-testable and works on a self-hosted box. Nights scale
clothing quantities, flights add liquids/power-bank/check-in, rentals add
licence + IDP, ferries add motion-sickness tablets, tropical stops add sun
cream and repellent, and the destination country picks the plug type from a
bundled ~50-country table. Every suggestion carries a short reason, and
already-added ones are keyed by suggestion_key so they can't be duplicated.

Two rules deliberately differ from the naive reading, both regression-tested:
a latitude floor stops a December trip to Bangkok being tagged cold as well
as tropical, and only a flight segment's arrival airport counts, since the
first segment's departure airport is home rather than a destination.

checklist_items is a new table, so the existing CREATE TABLE IF NOT EXISTS
path creates it on upgrade; no MIGRATIONS entry is needed and existing data
is untouched.

docs/API.md documents the full contract. 113/113 tests pass.

public/js/views/dragdrop.js

@@ -206,3 +206,19 @@ export function enableTripReorder(rows, refresh) {
     refresh,
   );
 }
+
+// ---------- Checklist: reorder items within a category ----------
+
+// `rows`: [{ node, handle, item }] — same mechanics as enableRowReorder,
+// scoped to one category's rows (checklist.js calls this once per category,
+// so the resulting sort_order values only need to rank correctly within that
+// category — category is always the primary sort key server-side).
+export function enableChecklistReorder(rows, refresh) {
+  enableReorder(
+    rows,
+    (item) => item.id,
+    (item) => item.sort_order,
+    (id, sort_order) => api.checklist.update(id, { sort_order }),
+    refresh,
+  );
+}