Add SSO login support

- Add [sso] config section with redirect_uri
- Create mcdsl/sso client when SSO is configured
- Add /login (landing page), /sso/redirect, /sso/callback routes
- Add /logout route
- Update login template with SSO landing page variant
- Bump mcdsl to v1.6.0

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

vendor/modernc.org/sqlite/unconvert.sh

@@ -0,0 +1,5 @@
+set -evx
+until unconvert -fastmath -all ./...
+do
+	unconvert2 -fastmath -apply -all ./...
+done