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>

docs/features/platform-homepage.md

@@ -67,8 +67,8 @@ app/
 │   └── content_page_service.py    # CMS business logic
 └── main.py                        # Platform routes (lines 284-404)
 
-scripts/
-└── create_platform_pages.py       # Seeder script
+scripts/seed/
+└── create_default_content_pages.py  # Seeder script (all platforms)
 
 docs/features/
 └── platform-homepage.md           # This file
@@ -193,14 +193,17 @@ Manage all platform content pages from a single interface:
 
 ```bash
 # Create all default platform pages
-python scripts/seed/create_platform_pages.py
+python scripts/seed/create_default_content_pages.py
 ```
 
-This creates:
+This creates (for all platforms):
 - Platform Homepage (with modern template)
 - About Us
-- FAQ
 - Contact Us
+- FAQ
+- Shipping Policy
+- Return & Refund Policy
+- Privacy Policy
 - Terms of Service
 - Privacy Policy
 
@@ -568,7 +571,7 @@ return templates.TemplateResponse(
 **Solutions:**
 1. Run seeder script:
    ```bash
-   python scripts/seed/create_platform_pages.py
+   python scripts/seed/create_default_content_pages.py
    ```
 
 2. Verify page exists: