feat(economy): tighten credit earning + deduct on request

Credit economy changes:
- creditsPerMinute: 10 -> 2 (45min episode = 90 COOP vs 450)
- movieRequestCost: 100 -> 500 (~5.5 episodes per movie)
- tvRequestCost: 200 -> 1000 (~11 episodes per season)
- tvPerSeasonCost: 50 -> 250

Race condition fix:
- Credits now deducted immediately at request time
- No more multiple requests before approval
- On DECLINED: automatic refund with ADJUSTMENT transaction
- On APPROVED: no extra deduction (already paid)

Updated schema defaults and all code fallbacks.

backend/src/routes/auth.ts

@@ -320,7 +320,7 @@ async function backfillUserHistory(user: {
 
 		// Get system settings for credit calculation
 		const settings = await prisma.systemSettings.findFirst();
-		const creditsPerMinute = settings?.creditsPerMinute || 10;
+		const creditsPerMinute = settings?.creditsPerMinute || 2;
 		const minWatchPercent = settings?.minWatchPercent || 80;
 		const minWatchMinutes = settings?.minWatchMinutes || 5;