fix: escrow sponsor/status index, dead stellar config removal, releasePartial/refund and SorobanClientService test coverage

[hardcordev]

Summary

Added the missing composite index on escrows(sponsorId, status), removed five dead Stellar config fields that were documented but never read, and filled two unit-test gaps: direct coverage for EscrowService.releasePartial()/refund() and a dedicated spec for SorobanClientService.

Changes

• Escrow.status has no explicit database index despite being filtered directly in hot query paths #97: Added @Index('IDX_escrow_sponsor_status', ['sponsorId', 'status']) on the Escrow entity plus a matching TypeORM migration (1784600000000-AddEscrowSponsorIdStatusIndex) serving SponsorsService.budgetLocked's sponsorId + status filter.
• Five AppConfig/stellar fields are populated from env vars but never read anywhere in the application #94: Removed the unused horizonUrl, maintenancePoolContractId, treasuryAddress, usdcAssetCode, and usdcAssetIssuer fields from AppConfig/configuration.ts, their .env.example entries (HORIZON_URL, MAINTENANCE_POOL_CONTRACT_ID, TREASURY_ADDRESS, USDC_ASSET_CODE, USDC_ASSET_ISSUER), and the corresponding README documentation, so operators are no longer told to configure values nothing reads.
• escrow.service.spec.ts has no test coverage for releasePartial() or refund() #96: Added describe('releasePartial') and describe('refund') blocks to escrow.service.spec.ts covering partial release staying LOCKED below the total, exact-completion flipping to RELEASED, a partial→partial cumulative completion sequence, over-request rejection with no chain call or writes, refund-when-not-LOCKED rejection, and a successful LOCKED→REFUNDED transition.
• SorobanClientService has no dedicated spec file — the real invoke/poll/toScVal logic is never unit tested #95: Added src/escrow/soroban-client.service.spec.ts exercising isConfigured() gating and dry-run invocation, the simulate → assemble → sign → send pipeline including simulation-failure and submission-error paths, pollTransaction's NOT_FOUND retry-then-resolve and timeout behavior via fake timers, and all four toScVal encoding branches (Stellar address, bigint/i128, plain string, number), using the existing stellar-sdk mock as the RPC double.

Issues

Resolves #97
Resolves #96
Resolves #95
Resolves #94

Verification

Verified by manual code review of the full diff against the service implementations, entity/migration conventions, jest config, and the existing stellar-sdk mock. cargo build and cargo test were not run.

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