fix: add uri strip_prefix /forge to Caddyfile generator — Forgejo routes 404 without it (completes #1080)

The /forge/* handle block in the Caddyfile generator was missing the
"uri strip_prefix /forge" directive. Without it, Caddy forwards
/forge/foo → forgejo:3000/forge/foo, Forgejo's router doesn't match,
and returns 404. Adding strip_prefix makes the prefix transparent to
Forgejo while ROOT_URL still governs link generation in HTML.

Same pattern as the /staging/* strip_prefix added in #1079.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/generators.sh

@@ -860,6 +860,7 @@ _generate_caddyfile_subpath() {
 
     # Reverse proxy to Forgejo
     handle /forge/* {
+        uri strip_prefix /forge
         reverse_proxy forgejo:3000
     }