Default WEBHOOKER_ENVIRONMENT to prod (closes #307)

An unset WEBHOOKER_ENVIRONMENT now means prod, not dev. The only
thing dev still changes is CORS, which then answers every origin with
Access-Control-Allow-Origin: *, so an operator who forgets the
variable is no longer silently permissive; dev must be set
explicitly. Cookie Secure and CSRF strictness follow each request's
transport and are unaffected.

The README, comments and tests no longer describe dev as the default:
the deployment checklist asks only that the environment is not dev,
the Docker and nginx examples drop the now-redundant setting, and the
TRUSTED_PROXIES warning gives its real reason for firing in every
environment.

Model: opus-4-8 (implementation); opus-5-5 (rework)

internal/reqtls/reqtls.go

@@ -5,7 +5,7 @@
 // several packages, by hand, and the answers disagreed. The session
 // cookie's Secure attribute was decided at startup from the configured
 // environment while the CSRF cookie's was decided per-request, so a
-// deployment behind a TLS proxy in the default environment emitted one
+// deployment behind a TLS proxy in the dev environment emitted one
 // Secure cookie and one non-Secure cookie on the same response.
 // Everything kept working, which is exactly why nobody noticed.
 //