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>

Makefile

@@ -104,22 +104,19 @@ init-prod:
 	@echo "Step 0/6: Ensuring database exists (running migrations)..."
 	@$(PYTHON) -m alembic upgrade heads
 	@echo ""
-	@echo "Step 1/6: Creating admin user and platform settings..."
+	@echo "Step 1/5: Creating admin user and platform settings..."
 	$(PYTHON) scripts/seed/init_production.py
 	@echo ""
-	@echo "Step 2/6: Initializing log settings..."
+	@echo "Step 2/5: Initializing log settings..."
 	$(PYTHON) scripts/seed/init_log_settings.py
 	@echo ""
-	@echo "Step 3/6: Creating default CMS content pages..."
+	@echo "Step 3/5: Creating default CMS content pages..."
 	$(PYTHON) scripts/seed/create_default_content_pages.py
 	@echo ""
-	@echo "Step 4/6: Creating platform pages and landing..."
-	$(PYTHON) scripts/seed/create_platform_pages.py
-	@echo ""
-	@echo "Step 5/6: Seeding email templates..."
+	@echo "Step 4/5: Seeding email templates..."
 	$(PYTHON) scripts/seed/seed_email_templates.py
 	@echo ""
-	@echo "Step 6/6: Seeding subscription tiers..."
+	@echo "Step 5/5: Seeding subscription tiers..."
 	@echo "  (Handled by init_production.py Step 6)"
 	@echo ""
 	@echo "✅ Production initialization completed"
@@ -195,10 +192,6 @@ create-cms-defaults:
 	$(PYTHON) scripts/seed/create_default_content_pages.py
 	@echo "✅ CMS defaults created"
 
-create-platform-pages:
-	@echo "🏠 Creating platform pages and landing..."
-	$(PYTHON) scripts/seed/create_platform_pages.py
-	@echo "✅ Platform pages created"
 
 init-logging:
 	@echo "📝 Initializing log settings..."