Everything on, phase 2 completed: SecureMode on in production, checked by the deploy

Owner decision 2026-10-04: SecureMode on once the pasture passes with it.

- Both clean pasture passes were run over all six peers:
  - normally: 246 passed, 0 failed;
  - with Federation__SecureMode=true: every federation check passed. The only failures were four checks expecting an
    unsigned GET to get 404 or 410 where SecureMode answers 401. Those checks now go through `unserved` and
    `gone_unsigned` (lib/interop.sh), which expect 401 when SecureMode is on.
- Circle posts now reach their member on GoToSocial and Mastodon, and survive Mastodon's signed refetch, as does a
  followers-only post. The GoToSocial expected failure is gone.
- appsettings.Production.json turns SecureMode on.
- The deploy now checks that an unsigned GET of @thepra answers 401 and that a browser is redirected. It reads
  @thepra's discoverability through the Mastodon API, since the actor is no longer readable unsigned.
- docs/INTEROP.md (Mastodon, GoToSocial), CLAUDE.md and ROADMAP updated.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01ELjqpznMFMNrJoJUj6K5p2

PrivaPub/appsettings.Production.json

@@ -5,6 +5,9 @@
 	"Registrations": {
 		"Mode": "Invitations"
 	},
+	"Federation": {
+		"SecureMode": true
+	},
 	"Statistics": {
 		"Crawler": {
 			"Enabled": true,