feat(categories): per-event category ordering — global default + override (#782)

Order a gallery's categories in the flow of the day instead of A–Z. Two layers,
resolved per event: per-event override > global default > name.

- migration 158: photo_categories.display_order (global default), backfilled
  from the current alphabetical order so existing galleries don't reshuffle.
- migration 159: event_category_order (event_id, category_id, position) — the
  per-event override; no backfill, every event starts on the default.
- utils/categoryOrder: shared resolution used by the admin event view and the
  public gallery; fails safe to the global default if the table is absent.
- adminCategories: POST /reorder sets a per-event override (globals +
  event-specific, interleaved); DELETE /reorder/:eventId resets; POST
  /reorder-global sets the global default. Ordering endpoints + create append.
- gallery renders the resolved order.
- Settings → Photo Categories reorders the global default; an event's Categories
  tab reorders that gallery (one combined list + Reset to default). Up/down
  buttons — no drag-and-drop dependency.
- en/de strings.

backend/src/utils/categoryOrder.js

@@ -0,0 +1,63 @@
+/**
+ * Category order resolution (#782).
+ *
+ * Resolves an event's categories into their effective display order, layering:
+ *   1. per-event override — event_category_order.position, when the event has
+ *      been customised;
+ *   2. the global default — photo_categories.display_order (migration 158);
+ *   3. name.
+ *
+ * Globals and event-specific categories are ordered together so a custom order
+ * can interleave them into the flow of the day. Shared by the admin event view
+ * and the public gallery so the two never diverge.
+ */
+const { db } = require('../database/db');
+const { formatBoolean } = require('./dbCompat');
+const { hasColumnCached } = require('./schemaCache');
+
+/**
+ * @param {number|string} eventId
+ * @param {object} [opts]
+ * @param {number[]|null} [opts.onlyIds] restrict to these category ids (the
+ *   public gallery only shows categories that actually have photos).
+ * @param {string[]|null} [opts.select] qualified columns to select (default
+ *   `c.*`). Always aliased to the `photo_categories as c` table.
+ * @returns rows with an added `override_position` (null when not customised).
+ */
+async function getEventCategoriesOrdered(eventId, { onlyIds = null, select = null } = {}) {
+  const eid = parseInt(eventId, 10);
+
+  const base = db('photo_categories as c').where(function () {
+    this.where('c.is_global', formatBoolean(true)).orWhere('c.event_id', eid);
+  });
+  if (onlyIds) base.whereIn('c.id', onlyIds);
+
+  // Fail safe: if the override table isn't present yet (half-applied migration),
+  // fall back to the global-default order so the public gallery never 500s.
+  const overrideReady = await hasColumnCached('event_category_order', 'position');
+  if (!overrideReady) {
+    return base
+      .select(select || 'c.*')
+      .orderBy('c.is_global', 'desc')
+      .orderBy('c.display_order', 'asc')
+      .orderBy('c.name', 'asc');
+  }
+
+  const cols = select ? [...select] : ['c.*'];
+  cols.push('o.position as override_position');
+
+  return base
+    .leftJoin('event_category_order as o', function () {
+      this.on('o.category_id', 'c.id').andOnVal('o.event_id', '=', eid);
+    })
+    .select(cols)
+    // Overridden categories first (in their pinned order), then the rest by the
+    // global default. CASE keeps NULL-ordering portable across SQLite + Postgres.
+    .orderByRaw('CASE WHEN o.position IS NULL THEN 1 ELSE 0 END ASC')
+    .orderBy('o.position', 'asc')
+    .orderBy('c.is_global', 'desc')
+    .orderBy('c.display_order', 'asc')
+    .orderBy('c.name', 'asc');
+}
+
+module.exports = { getEventCategoriesOrdered };