feat: integrate PlatformContextMiddleware and update routes for multi-platform

Phase 2 implementation:
- Register PlatformContextMiddleware in main.py (runs before VendorContextMiddleware)
- Update VendorContextMiddleware to use platform_clean_path
- Update platform homepage route to use three-tier CMS resolution
- Update platform content page routes with platform context
- Update vendor root path handlers with platform_id support

Middleware execution order:
1. LoggingMiddleware
2. PlatformContextMiddleware (detect platform from domain/path)
3. VendorContextMiddleware (detect vendor, uses platform_clean_path)
4. ContextMiddleware
5. LanguageMiddleware
6. ThemeContextMiddleware

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

middleware/vendor_context.py

@@ -9,6 +9,9 @@ Handles three routing modes:
 3. Path-based (/vendor/vendor1/ or /vendors/vendor1/ → Vendor 1)
 
 Also extracts clean_path for nested routing patterns.
+
+IMPORTANT: This middleware runs AFTER PlatformContextMiddleware.
+Uses request.state.platform_clean_path when available (set by PlatformContextMiddleware).
 """
 
 import logging
@@ -39,10 +42,14 @@ class VendorContextManager:
         2. Subdomain (vendor1.platform.com)
         3. Path-based (/vendor/vendor1/ or /vendors/vendor1/)
 
+        Uses platform_clean_path from PlatformContextMiddleware when available.
+        This path has the platform prefix stripped (e.g., /oms/vendors/foo → /vendors/foo).
+
         Returns dict with vendor info or None if not found.
         """
         host = request.headers.get("host", "")
-        path = request.url.path
+        # Use platform_clean_path if available (set by PlatformContextMiddleware)
+        path = getattr(request.state, "platform_clean_path", None) or request.url.path
 
         # Remove port from host if present (e.g., localhost:8000 -> localhost)
         if ":" in host:
@@ -393,7 +400,9 @@ class VendorContextMiddleware(BaseHTTPMiddleware):
     - More organized code
     - Standard ASGI pattern
 
-    Runs FIRST in middleware chain.
+    Runs AFTER PlatformContextMiddleware in the request chain.
+    Uses request.state.platform_clean_path for path-based vendor detection.
+
     Sets:
         request.state.vendor: Vendor object
         request.state.vendor_context: Detection metadata