refactor(db): split schema into domain modules

Split the 276-line schema.ts into focused domain modules:
- users.ts: classes, users, userTimers (core identity)
- inventory.ts: items, inventory (item system)
- economy.ts: transactions, itemTransactions (currency flow)
- quests.ts: quests, userQuests (quest system)
- moderation.ts: moderationCases, lootdrops (moderation)

Original schema.ts now re-exports from schema/index.ts for backward
compatibility. All existing imports continue to work.

shared/db/schema/index.ts

@@ -0,0 +1,6 @@
+// Domain modules
+export * from './users';
+export * from './inventory';
+export * from './economy';
+export * from './quests';
+export * from './moderation';