feat: multi-person assignment for tasks and calendar events

- DB migration v32: task_assignments and event_assignments join tables
  with CASCADE delete; existing assigned_to data migrated automatically
- Tasks API: accepts assigned_to as array, returns assigned_users[]
  with json_group_array; filter uses EXISTS on task_assignments
- Calendar API: same pattern via event_assignments; serializeEvent
  includes assigned_users array
- Recurring task completion copies all assignments to the new instance
- Frontend: shared UserMultiSelect component with avatar stack display
  (renderAvatarStack, renderUserMultiSelect, getSelectedUserIds,
  bindUserMultiSelect); tasks.js and calendar.js use it in modals
  and card/agenda views
- CSS: user-multi-select.css with avatar-stack and user-ms classes
- 14 new tests covering CRUD, JSON aggregation, EXISTS filter,
  and CASCADE behavior for both task and event assignments

Closes #125

server/db.js

@@ -1189,6 +1189,26 @@ const MIGRATIONS = [
       ALTER TABLE birthdays ADD COLUMN reminder_custom_unit TEXT;
     `,
   },
+  {
+    version: 32,
+    description: 'Multi-person assignment for tasks and calendar events',
+    up: `
+      CREATE TABLE IF NOT EXISTS task_assignments (
+        task_id  INTEGER NOT NULL REFERENCES tasks(id) ON DELETE CASCADE,
+        user_id  INTEGER NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+        PRIMARY KEY (task_id, user_id)
+      );
+      CREATE TABLE IF NOT EXISTS event_assignments (
+        event_id INTEGER NOT NULL REFERENCES calendar_events(id) ON DELETE CASCADE,
+        user_id  INTEGER NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+        PRIMARY KEY (event_id, user_id)
+      );
+      INSERT OR IGNORE INTO task_assignments (task_id, user_id)
+        SELECT id, assigned_to FROM tasks WHERE assigned_to IS NOT NULL;
+      INSERT OR IGNORE INTO event_assignments (event_id, user_id)
+        SELECT id, assigned_to FROM calendar_events WHERE assigned_to IS NOT NULL;
+    `,
+  },
 ];
 
 /**