feat(routes): register tenants and feature-flag routers in the app

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Closes #1216

Summary

Registers createTenantsRouter at /tenants (/api/tenants) and createFeatureFlagsRouter at /feature-flags (/api/feature-flags) inside createApiRouter (src/routes/index.ts), allows app.locals.tenantRepository overrides in src/routes/tenants.ts, updates src/routes/tenants.openapi.test.ts to run against createApp(), and documents the app registration in docs/tenants-api.md.

Acceptance Criteria Mapping

• POST /api/tenants, GET /api/tenants, and PATCH /api/tenants/:tenantId are reachable on createApp() and enforce authentication + Zod validation: Verified in src/routes/tenants.openapi.test.ts.
• GET /api/feature-flags is reachable on createApp() and returns the enveloped flag map with an ETag header and 304 support: Verified in src/routes/tenants.openapi.test.ts.
• src/routes/tenants.openapi.test.ts runs against createApp(): Updated buildApp to use createApp().

Validation

• npx jest src/routes/tenants.openapi.test.ts --no-cache (all 23 tests pass).

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