fix(app-server): attach API middleware stack

[OnlyYu1996]

Summary

• attach the existing app-server rate-limit, content-type, timeout, and security-header middleware to /api/v1 routes
• reuse the configured CORS layer instead of always using permissive CORS
• preserve existing Axum 0.7-style route syntax compatibility so the app-server router can be constructed under Axum 0.8
• add router-level regression coverage for rate limiting, timeout, content-type rejection, and security headers

Fixes PlatformNetwork/bounty-challenge#53530.

Validation

• cargo +stable fmt --package cortex-app-server --check
• cargo +stable test --package cortex-app-server create_router_applies -- --nocapture
• cargo +stable check --package cortex-app-server --lib
• git diff --check

[greptile-apps]

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