feat: strategy cooldown, withdrawal receipts, vault caching, gasless deposits

[OtowoSamuel]

Summary

Implements 4 features across smart contracts, backend, and frontend:

#1237 — Strategy Switching Cooldown Period

• Contract: Added StrategySwitchCooldown and LastStrategySwitchTime storage keys
• Contract: Cooldown enforcement in set_strategy and execute_strategy_proposal
• Contract: Admin-configurable via set_strategy_switch_cooldown(seconds)
• Contract: Read-only strategy_switch_cooldown_remaining() view
• Contract: Events: stratcd (cooldown check), stratset (strategy changed)
• Backend: GET /vault/strategy/cooldown endpoint with remaining time
• Backend: Cooldown enforcement on POST /vault/strategy (429 on cooldown)
• Frontend: Cooldown indicator in VaultDashboard strategy section
• Frontend: VaultContext tracks and polls cooldown state

#1236 — Detailed Withdrawal Receipt

• Backend: Receipt data included in deposit/withdrawal response bodies
• Backend: GET /vault/receipts endpoint with pagination and wallet filter
• Backend: Explorer link generation (Stellar Expert + Horizon API)
• Frontend: Enhanced TransactionReceipt page with:
  • Status polling for pending transactions
  • Confirmation count and ledger number display
  • Stellar Expert and Horizon API links
  • JSON export functionality
  • Retry logic for unconfirmed transactions

#1253 — Backend Caching Layer

• Backend: New vaultDataCache.ts module with dedicated caching for:
  • Vault summaries (30s TTL)
  • Strategy performance (60s TTL)
  • User stats (15s TTL)
  • Share price (10s TTL)
• Backend: Cache invalidation hooks registered for transaction events
• Backend: Prometheus metrics for cache hit/miss tracking
• Backend: Automatic fallthrough to database on cache miss

#1234 — Gasless Deposits (Soroban-Native)

Note: This is a Soroban/Stellar project, not EVM. ERC-20 permit doesn't apply.
Implemented the Soroban-native equivalent: off-chain signed intent + relayer submission.

• Contract: gasless_deposit(relayer, user, amount) entrypoint
• Contract: GaslessRelayer storage key and set_gasless_relayer admin function
• Contract: glessdep event on successful gasless deposit
• Backend: POST /vault/gasless-deposits endpoint
• Frontend: useGaslessDeposit hook with signing → submitting → confirming flow

Closes

Closes #1237
Closes #1236
Closes #1253
Closes #1234

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