feat: complete multi-platform CMS phases 2-5

Phase 2 - OMS Migration & Integration:
- Fix platform_pages.py to use get_platform_page for marketing pages
- Fix shop_pages.py to pass platform_id to content page service calls

Phase 3 - Admin Interface:
- Add platform management API (app/api/v1/admin/platforms.py)
- Add platforms admin page with stats cards
- Add Platforms menu item to admin sidebar
- Update content pages admin with platform filter and four-tab tier system

Phase 4 - Documentation:
- Add comprehensive architecture docs (docs/architecture/multi-platform-cms.md)
- Update implementation plan with completion status

Phase 5 - Vendor Dashboard:
- Add CMS usage API endpoint with tier limits
- Add usage progress bar to vendor content pages
- Add platform-default/{slug} API for preview
- Add View Default button and modal in page editor

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/routes/admin_pages.py

@@ -1039,6 +1039,78 @@ async def admin_logs_page(
     )
 
 
+# ============================================================================
+# PLATFORM MANAGEMENT ROUTES (Multi-Platform Support)
+# ============================================================================
+
+
+@router.get("/platforms", response_class=HTMLResponse, include_in_schema=False)
+async def admin_platforms_list(
+    request: Request,
+    current_user: User = Depends(get_current_admin_from_cookie_or_header),
+    db: Session = Depends(get_db),
+):
+    """
+    Render platforms management page.
+    Shows all platforms (OMS, Loyalty, etc.) with their configuration.
+    """
+    return templates.TemplateResponse(
+        "admin/platforms.html",
+        {
+            "request": request,
+            "user": current_user,
+        },
+    )
+
+
+@router.get(
+    "/platforms/{platform_code}", response_class=HTMLResponse, include_in_schema=False
+)
+async def admin_platform_detail(
+    request: Request,
+    platform_code: str = Path(..., description="Platform code (oms, loyalty, etc.)"),
+    current_user: User = Depends(get_current_admin_from_cookie_or_header),
+    db: Session = Depends(get_db),
+):
+    """
+    Render platform detail page.
+    Shows platform configuration, marketing pages, and vendor defaults.
+    """
+    return templates.TemplateResponse(
+        "admin/platform-detail.html",
+        {
+            "request": request,
+            "user": current_user,
+            "platform_code": platform_code,
+        },
+    )
+
+
+@router.get(
+    "/platforms/{platform_code}/edit",
+    response_class=HTMLResponse,
+    include_in_schema=False,
+)
+async def admin_platform_edit(
+    request: Request,
+    platform_code: str = Path(..., description="Platform code"),
+    current_user: User = Depends(get_current_admin_from_cookie_or_header),
+    db: Session = Depends(get_db),
+):
+    """
+    Render platform edit form.
+    Allows editing platform settings, branding, and configuration.
+    """
+    return templates.TemplateResponse(
+        "admin/platform-edit.html",
+        {
+            "request": request,
+            "user": current_user,
+            "platform_code": platform_code,
+        },
+    )
+
+
 # ============================================================================
 # CONTENT MANAGEMENT SYSTEM (CMS) ROUTES
 # ============================================================================