fix: consolidate CMS page seed scripts and fix 3 bugs

- Fix `ContentPage.store_id is None` (Python identity check, always
  False) → use `.is_(None)` for proper SQLAlchemy NULL filtering
- Create pages for ALL platforms instead of only OMS
- Merge create_platform_pages.py into create_default_content_pages.py
  (5 overlapping pages, only platform_homepage was unique)
- Delete redundant create_platform_pages.py
- Update Makefile, install.py, and docs references

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

scripts/seed/install.py

@@ -543,8 +543,7 @@ def main():
     init_scripts = [
         ("init_production.py", "Admin user and platform settings"),
         ("init_log_settings.py", "Log settings"),
-        ("create_default_content_pages.py", "Default CMS pages"),
-        ("create_platform_pages.py", "Platform pages and landing"),
+        ("create_default_content_pages.py", "Default CMS pages (all platforms)"),
         ("seed_email_templates.py", "Email templates"),
     ]