feat: Add site title and site description on PLG onboarding

docs/db/mysticat-plg-onboarding-site-metadata.sql

@@ -0,0 +1,3 @@
+-- Superseded by mysticat-data-service migration:
+--   db/migrations/20260330100013_plg_onboardings_site_metadata.sql
+-- (includes postgrest_writer column grants). Kept for historical reference only.

docs/openapi/schemas.yaml

@@ -6792,6 +6792,18 @@ PlgOnboarding:
         - 'null'
       description: Reason for waitlisting, if applicable
       example: null
+    siteTitle:
+      type:
+        - string
+        - 'null'
+      description: Site title
+      example: 'Example Corp'
+    siteDescription:
+      type:
+        - string
+        - 'null'
+      description: Site description
+      example: 'Official site for Example Corp products and support.'
     completedAt:
       $ref: '#/DateTime'
     createdAt: