fix: use NOW() in weighted score query instead of JS Date parameter

Passing a JS Date to a Drizzle sql template serializes it as a locale
string (e.g. "Mon Mar 09 2026 19:51:22 GMT+0100") which PostgreSQL
cannot parse as timestamptz, causing the gamification worker to fail.

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

packages/backend/src/lib/gamification.ts

@@ -60,10 +60,9 @@ export async function revokePoints(
 }
 
 export async function calculateWeightedScore(db: DB, userId: string): Promise<number> {
-  const now = new Date();
   const result = await db.execute(sql`
     SELECT SUM(
-      points * EXP(-${DECAY_LAMBDA} * EXTRACT(EPOCH FROM (${now}::timestamptz - date_created)) / 86400)
+      points * EXP(-${DECAY_LAMBDA} * EXTRACT(EPOCH FROM (NOW() - date_created)) / 86400)
     ) as weighted_score
     FROM user_points
     WHERE user_id = ${userId}