feat(blog): add subscription-only content gating

Adds SubscriptionOnly flag to blog posts to restrict full content access to active subscribers. Shows teaser/preview for non-subscribers with subscribe CTA. Integrates with repository subscription system when monetization is enabled. Updates v2 API structs and editor UI with subscription toggle. Admins and repo writers bypass the gate.

services/forms/blog_form.go

@@ -14,14 +14,15 @@ import (
 
 // BlogPostForm is the form for creating/editing blog posts.
 type BlogPostForm struct {
-	Title         string `binding:"Required;MaxSize(255)"`
-	Subtitle      string `binding:"MaxSize(500)"`
-	Content       string `binding:"Required"`
-	Tags          string `binding:"MaxSize(1000)"`
-	Series        string `binding:"MaxSize(255)"`
-	FeaturedImage string // attachment UUID
-	Status        int    `binding:"Range(0,2)"` // 0=draft, 1=public, 2=published
-	AllowComments bool
+	Title            string `binding:"Required;MaxSize(255)"`
+	Subtitle         string `binding:"MaxSize(500)"`
+	Content          string `binding:"Required"`
+	Tags             string `binding:"MaxSize(1000)"`
+	Series           string `binding:"MaxSize(255)"`
+	FeaturedImage    string // attachment UUID
+	Status           int    `binding:"Range(0,2)"` // 0=draft, 1=public, 2=published
+	AllowComments    bool
+	SubscriptionOnly bool
 }
 
 // Validate validates the fields