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/exceptions/platform.py

@@ -0,0 +1,44 @@
+# app/exceptions/platform.py
+"""
+Platform-related exceptions.
+
+Custom exceptions for platform management operations.
+"""
+
+from app.exceptions.base import WizamartException
+
+
+class PlatformNotFoundException(WizamartException):
+    """Raised when a platform is not found."""
+
+    def __init__(self, code: str):
+        super().__init__(
+            message=f"Platform not found: {code}",
+            error_code="PLATFORM_NOT_FOUND",
+            status_code=404,
+            details={"platform_code": code},
+        )
+
+
+class PlatformInactiveException(WizamartException):
+    """Raised when trying to access an inactive platform."""
+
+    def __init__(self, code: str):
+        super().__init__(
+            message=f"Platform is inactive: {code}",
+            error_code="PLATFORM_INACTIVE",
+            status_code=403,
+            details={"platform_code": code},
+        )
+
+
+class PlatformUpdateException(WizamartException):
+    """Raised when platform update fails."""
+
+    def __init__(self, code: str, reason: str):
+        super().__init__(
+            message=f"Failed to update platform {code}: {reason}",
+            error_code="PLATFORM_UPDATE_FAILED",
+            status_code=400,
+            details={"platform_code": code, "reason": reason},
+        )