fix: wallet balance cache, Stellar secret validation, destination check, WalletsService cleanup

[prismn]

Summary

Changes

#869 — Wallet balance cache migration

• Replaced in-memory Map with Redis-backed cache via global CacheModule
• Cache key: wallet-balances:${userId} with configurable TTL (WALLET_BALANCE_CACHE_TTL_SECONDS, default 30s)
• Cache invalidated on successful deposit/withdrawal via UsersService.invalidateWalletBalanceCache()

#868 — Secure Stellar secret key handling

• Removed (this as any).configService?.get() pattern — properly typed ConfigService injection
• Added onModuleInit() that validates STELLAR_HOT_WALLET_SECRET at startup (fail-fast)
• Added getStellarSecretKey() with proper error handling
• Made STELLAR_HOT_WALLET_SECRET a required field in Zod env validation schema

#867 — Eliminate JSONB balance bypass

• Fixed duplicate variable declarations and duplicate method signature in WalletsService (compile errors)
• All balance operations already go through WalletsService — no remaining user.balances JSONB access
• WalletsService.adjustBalance already uses pessimistic_write row-level locking

#789 — Destination account validation

• Added StellarService.accountExists(address) — queries Horizon /accounts/{address}
• Result cached in Redis for 60 seconds per address
• createWithdrawal checks destination before proceeding; returns 400 with clear message if unfunded
• Added destinationAddress field to WithdrawalDto

Files Changed

• src/config/env.validation.ts — required STELLAR_HOT_WALLET_SECRET, added WALLET_BALANCE_CACHE_TTL_SECONDS
• src/users/users.service.ts — Redis-backed wallet balance cache with invalidation
• src/wallet/wallets.service.ts — fixed duplicate declarations (compile errors)
• src/stellar/stellar.service.ts — added accountExists() with Redis caching
• src/transactions/transactions.service.ts — ConfigService injection, getStellarSecretKey, destination check, cache invalidation
• src/transactions/transactions.module.ts — imported StellarModule

Closes #869
Closes #868
Closes #867
Closes #789

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[portableDD]

@prismn please resolve conflicts

[prismn]

@portableDD it has been fixed, tanks