feat(routes): mount forecast router on API index (#1215)

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Summary

Mounts the imported forecast routers in src/routes/index.ts at /api/forecast and /api/billing/forecast with consistent authentication and response shapes, and documents the canonical endpoint and alias in docs/billing-forecast-endpoint.md.

Closes #1215

Key Changes

1. Router Mounting (src/routes/index.ts):
   • Mounted createForecastRouter() at /forecast (GET /api/forecast) and createBillingForecastRouter() at /billing/forecast (GET /api/billing/forecast).
2. Authentication & Response Parity (src/routes/forecast.ts, src/routes/billing/forecast.ts):
   • Added requireAuth fallback support on GET /api/forecast when no upstream proxy injected req.user while preserving direct req.user test injection.
   • Exported createBillingForecastRouter and aligned docs/billing-forecast-endpoint.md with the mounted routes.
3. Integration Tests (src/routes/forecast.test.ts):
   • Added integration tests against createApp() verifying 401 when unauthenticated and 200 with projected spend on both /api/forecast and /api/billing/forecast.

Verification

• Verified with npx jest src/routes/forecast.test.ts (36/36 passed).

Payout Wallet (Base Mainnet USDC)

0x3806cd592a508d1992Ed007C2F53de251C2E7452