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>

static/admin/js/platforms.js

@@ -4,6 +4,8 @@
  * Handles platform listing and management for multi-platform CMS.
  */
 
+const platformsLog = window.LogConfig.createLogger('PLATFORMS');
+
 function platformsManager() {
   return {
     // State
@@ -13,6 +15,10 @@ function platformsManager() {
 
     // Lifecycle
     async init() {
+      // Duplicate initialization guard
+      if (window._adminPlatformsInitialized) return;
+      window._adminPlatformsInitialized = true;
+
       this.currentPage = "platforms";
       await this.loadPlatforms();
     },
@@ -25,9 +31,9 @@ function platformsManager() {
       try {
         const response = await apiClient.get("/admin/platforms");
         this.platforms = response.platforms || [];
-        console.log(`[PLATFORMS] Loaded ${this.platforms.length} platforms`);
+        platformsLog.info(`Loaded ${this.platforms.length} platforms`);
       } catch (err) {
-        console.error("[PLATFORMS] Error loading platforms:", err);
+        platformsLog.error("Error loading platforms:", err);
         this.error = err.message || "Failed to load platforms";
       } finally {
         this.loading = false;