Server returns 200 (soft 404) for non-existent routes instead of a real 404

[Alek99]

Describe the bug

Reflex apps return HTTP 200 for URLs that don't correspond to any real page, serving the SPA fallback as the response body. This is a soft 404: the page doesn't exist, but the server signals success. Crawlers, AI agents, and SEO tooling all treat the response as a valid page and try to extract content from what is effectively an error state.

The root cause is that the SPA fallback is doing double duty. It's how Reflex resolves dynamic routes and other runtime-resolved paths and it's what gets served when a URL doesn't match anything. Both cases hit the same fallback, so the server can't distinguish "valid dynamic route" from "nonexistent page"

We need to be able to distinguish between the two

To Reproduce

1. Deploy any Reflex app.
2. Request a URL that doesn't map to a defined page, e.g. /this-page-does-not-exist.
3. Observe: response is 200 OK with the SPA fallback HTML.
4. Expected: 404 Not Found.

[linear]

ENG-9472

[adhami3310]

on what level would you like this to be resolved? as it's typical for people in a production build to put something like Caddy/nginx, and since we produce static frontend we have no script to run there. it depends on the deployment