fix(proxy): require balance covering endpoint price before proxying

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Closes #1224

Summary

Resolves the matched endpoint's priceUsdc before the pre-proxy balance check in handleProxy (src/routes/proxyRoutes.ts), rejects requests with 402 Payment Required (INSUFFICIENT_BALANCE in the standard error envelope) without contacting the upstream service when currentBalance < endpointPriceUsdc, and allows free endpoints (priceUsdc === 0) to proceed even when the caller's balance is zero.

Acceptance Criteria Mapping

• A user with balance 1 calling a 5-price endpoint receives 402 and the upstream is not called: Verified in src/routes/gatewayRoutes.test.ts and src/__tests__/proxy.integration.test.ts.
• A user with balance 5 calling a 5-price endpoint succeeds and has 5 deducted: Verified in src/routes/gatewayRoutes.test.ts and src/__tests__/proxy.integration.test.ts.
• A user with balance 0 calling a 0-price endpoint succeeds: Free endpoints skip the positive-balance requirement and proxy normally.
• Error response matches the standard error envelope: Throws new PaymentRequiredError('Payment Required: insufficient balance', 'INSUFFICIENT_BALANCE').

Validation

• npx jest src/routes/gatewayRoutes.test.ts --no-cache and npx jest src/__tests__/proxy.integration.test.ts -t "1224" --no-cache (all pass).

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