Move auth server from federation to account

Account is now the OAuth authorization server with magic link login,
OAuth2 authorize endpoint, SSO logout, and session management.

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

templates/_email/magic_link.txt

@@ -0,0 +1,8 @@
+Hello,
+
+Click this link to sign in:
+{{ link_url }}
+
+This link will expire in 15 minutes.
+
+If you did not request this, you can ignore this email.