Declare typed Convex env and route auth through @code/env/convex

- Register SITE_URL, NATIVE_APP_URL, CONVEX_SITE_URL, FLUE_DB_TOKEN in
  convex.config.ts so the generated server.js exposes a typed env object.
- Rewrite auth.ts to consume @code/env/convex and tighten the
  getCurrentUser handler.
- Refresh the backend README to describe the control-plane role and
  generated-file policy.

packages/backend/convex/_generated/server.js

@@ -91,3 +91,8 @@ export const internalAction = internalActionGeneric;
  * @returns The wrapped function. Import this function from `convex/http.js` and route it to hook it up.
  */
 export const httpAction = httpActionGeneric;
+
+/**
+ * Typesafe environment variables declared in `convex.config.ts`.
+ */
+export const env = process.env;