fix: resolve architecture validation errors

- Create platform_service.py to move DB queries from platforms.py API
- Create platform.py exceptions for PlatformNotFoundException
- Update platforms.py API to use platform_service
- Update vendor/content_pages.py to use vendor_service
- Add get_vendor_by_id_optional method to VendorService
- Fix platforms.js: add centralized logger and init guard
- Fix content-page-edit.html: use modal macro instead of inline modal

All 21 architecture validation errors resolved.

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

app/services/vendor_service.py

@@ -257,6 +257,19 @@ class VendorService:
 
         return vendor
 
+    def get_vendor_by_id_optional(self, db: Session, vendor_id: int) -> Vendor | None:
+        """
+        Get vendor by ID, returns None if not found.
+
+        Args:
+            db: Database session
+            vendor_id: Vendor ID to find
+
+        Returns:
+            Vendor object or None if not found
+        """
+        return db.query(Vendor).filter(Vendor.id == vendor_id).first()
+
     def get_active_vendor_by_code(self, db: Session, vendor_code: str) -> Vendor:
         """
         Get active vendor by vendor_code for public access (no auth required).