Feat: integrate backend with Soroban contract

[ebubechi-ihediwa]

Wire backend prediction logic to Soroban smart contract for hybrid on-chain/off-chain architecture.

Core Integration:

• Implement Predict (place_bet) and Resolve (resolve_round) contract calls via soroban.service.ts
• Add createRound() for on-chain round creation with price/duration validation
• Mode-aware interaction: UP_DOWN fully integrated, LEGENDS returns 501 Not Implemented
• Centralize all blockchain operations with structured error handling and retry logic

Error Handling & Resilience:

• Structured error classification (TRANSIENT, VALIDATION, INSUFFICIENT_FUNDS, CONTRACT_ERROR, TIMEOUT, UNKNOWN)
• Automatic retry with exponential backoff for network failures (3 retries, 30s timeout)
• Database rollback on blockchain failure during critical operations
• Graceful degradation: user payouts complete even if blockchain unavailable
• Clear error surfacing with proper HTTP status codes (400, 503, 504)

Testing:

• E2E test suite (18 tests) validates complete round lifecycle
• Mock blockchain client enables testing without deployed contract
• Tests cover: round creation, predictions, resolution, errors, authorization
• Database-only mode for CI/CD environments

Configuration:

• SOROBAN_CONTRACT_ID, SOROBAN_NETWORK, SOROBAN_RPC_URL
• SOROBAN_ADMIN_SECRET (round operations), SOROBAN_ORACLE_SECRET (price resolution)
• Service gracefully degrades when Soroban not configured

Files Modified:

• src/services/soroban.service.ts (blockchain integration layer)
• src/services/round.service.ts (integrated round creation)
• src/services/prediction.service.ts (integrated bet placement)
• src/services/resolution.service.ts (integrated resolution)
• src/routes/rounds.routes.ts, predictions.routes.ts (error handling)
• src/tests/soroban-integration.e2e.test.ts (test suite)
• src/tests/setup.ts (test utilities)
• src/types/xelma-bindings.d.ts (type declarations)
• src/middleware/auth.middleware.ts (fixed duplicates)

Closes #24