fix(egress): emit preserve_auth into the generated routes

The flag from #453 was parsed off the manifest and honored by the
addon, but `_route_to_yaml_fields` never wrote it, so it was dropped on
the way to the proxy and has never worked end to end. Every registry
route silently kept the unconditional Authorization strip.

That is the whole of the Docker Hub / GHCR "unauthorized" blocker in
#392: podman fetched a valid 2658-byte bearer from auth.docker.io, the
proxy stripped it, and registry-1.docker.io answered the resulting
anonymous request with a fresh WWW-Authenticate challenge — which reads
exactly like a credential problem rather than a serializer gap.

Found by diffing a bottle's rendered routes.yaml against its manifest:
preserve_auth was in the manifest and absent from the output.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

bot_bottle/egress.py

@@ -255,6 +255,8 @@ def _route_to_yaml_fields(r: Route) -> dict[str, object]:
         fields["matches"] = matches_data
     if r.git_fetch:
         fields["git"] = {"fetch": True}
+    if r.preserve_auth:
+        fields["preserve_auth"] = True
     if (
         r.outbound_detectors is not None
         or r.inbound_detectors is not None
@@ -335,6 +337,8 @@ def egress_render_routes(
             lines.append("    git:")
             if git_dict.get("fetch") is True:
                 lines.append("      fetch: true")
+        if f.get("preserve_auth") is True:
+            lines.append("    preserve_auth: true")
         if "dlp" in f:
             dlp_dict: dict[str, object] = f["dlp"]  # type: ignore
             lines.append("    dlp:")