feat: add Activity type system, DB migration, and self-only showActivity in sanitizeUser

- Add Activity, ActivityType, ActivityTimestamps, ActivityAssets types to shared types
- Add activity_update client event and activities field on presence_update server event
- Add userActivities to ready payload and showActivity to User/UpdateUserRequest
- Create shared activities.ts with ACTIVITY_LIMITS, ACTIVITY_PRIORITY, getPrimaryActivity
- Add show_activity column to users table (schema + migration)
- Update sanitizeUser with isSelf parameter; only include showActivity for self
- Fix .map(sanitizeUser) calls to use arrow wrapper to prevent index-as-boolean bug
- Mark auth routes (register/login) as isSelf=true since they return own user data

packages/shared/src/activities.ts

@@ -0,0 +1,25 @@
+import type { Activity, ActivityType } from './types.js';
+
+export const ACTIVITY_LIMITS = {
+  MAX_ACTIVITIES_PER_USER: 5,
+  MAX_NAME_LENGTH: 128,
+  MAX_DETAILS_LENGTH: 128,
+  MAX_STATE_LENGTH: 128,
+  MAX_ASSET_TEXT_LENGTH: 128,
+  MAX_URL_LENGTH: 512,
+} as const;
+
+export const ACTIVITY_PRIORITY: Record<ActivityType, number> = {
+  streaming: 5,
+  playing: 4,
+  listening: 3,
+  watching: 2,
+  custom: 1,
+};
+
+export function getPrimaryActivity(activities: Activity[]): Activity | null {
+  if (!activities.length) return null;
+  return activities.reduce((best, current) =>
+    ACTIVITY_PRIORITY[current.type] > ACTIVITY_PRIORITY[best.type] ? current : best
+  );
+}