fix: prevent achievement points from being re-awarded on republish

Once an achievement is unlocked, preserve date_unlocked permanently
instead of clearing it to null when the user drops below the threshold
(e.g. on unpublish). This prevents the wasUnlocked check from returning
false on republish, which was causing achievement points to be re-awarded
on every publish/unpublish cycle.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/backend/src/lib/gamification.ts

@@ -195,7 +195,9 @@ export async function checkAchievements(db: DB, userId: string, category?: strin
         .update(user_achievements)
         .set({
           progress,
-          date_unlocked: isUnlocked ? existing[0].date_unlocked || new Date() : null,
+          date_unlocked: isUnlocked
+            ? (existing[0].date_unlocked ?? new Date())
+            : existing[0].date_unlocked,
         })
         .where(
           and(