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/prisma/schema.prisma

@@ -152,14 +152,14 @@ model SystemSettings {
   id          String   @id @default(cuid())
   
   // Minting settings
-  creditsPerMinute Int @default(10) @map("credits_per_minute")
+  creditsPerMinute Int @default(2) @map("credits_per_minute")
   minWatchPercent  Int @default(80) @map("min_watch_percent")
   minWatchMinutes  Int @default(5) @map("min_watch_minutes")
   
   // Spending settings
-  movieRequestCost Int @default(100) @map("movie_request_cost")
-  tvRequestCost    Int @default(200) @map("tv_request_cost")
-  tvPerSeasonCost  Int @default(50) @map("tv_per_season_cost")
+  movieRequestCost Int @default(500) @map("movie_request_cost")
+  tvRequestCost    Int @default(1000) @map("tv_request_cost")
+  tvPerSeasonCost  Int @default(250) @map("tv_per_season_cost")
   
   // Multipliers
   newReleaseMultiplier Decimal @default(1.5) @map("new_release_multiplier") @db.Decimal(3, 2)