feat(vault): persist vault records in Postgres

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Closes #1208

Summary

Implements PgVaultRepository in src/repositories/vaultRepository.ts backed by pg.Pool (mapping unique constraint violations to DuplicateVaultError and missing rows to VaultNotFoundError), updates migrations/0001_create_api_keys_and_vaults.up.sql so vaults.user_id is TEXT NOT NULL and vaults.balance_snapshot is BIGINT NOT NULL DEFAULT 0, wires PgVaultRepository(pool) in src/app.ts when DATABASE_URL is configured, and runs the repository test suite against both InMemoryVaultRepository and PgVaultRepository (pg-mem).

Acceptance Criteria Mapping

• create persists a row in vaults and throws DuplicateVaultError on duplicate (user_id, network): Enforced via UNIQUE (user_id, network) and mapped in PgVaultRepository.create.
• findByUserId returns the persisted record across a fresh repository instance: Verified in src/repositories/vaultRepository.test.ts.
• updateBalanceSnapshot updates balance_snapshot, last_synced_at, and updated_at, and throws VaultNotFoundError for unknown IDs: Implemented and verified in src/repositories/vaultRepository.test.ts.
• Both InMemoryVaultRepository and PgVaultRepository pass the shared contract test suite: Verified in src/repositories/vaultRepository.test.ts.

Validation

• npx jest src/repositories/vaultRepository.test.ts --no-cache (all 20 tests pass).

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Payout Wallet (Base Mainnet USDC / EVM): 0x3806cd592a508d1992Ed007C2F53de251C2E7452