Add LinkedIn OAuth connect with portal-saved app credentials.

Replace manual token paste with Authorization Code flow; store Client ID/secret in the DB from the social accounts UI and surface a copyable callback URL for LinkedIn Auth setup.

site/social/admin.py

@@ -1,6 +1,6 @@
 from django.contrib import admin
 
-from social.models import SocialAccount, SocialPost, SocialPostTarget
+from social.models import SocialAccount, SocialAppCredentials, SocialPost, SocialPostTarget
 
 
 class SocialPostTargetInline(admin.TabularInline):
@@ -8,6 +8,12 @@ class SocialPostTargetInline(admin.TabularInline):
     extra = 0
 
 
+@admin.register(SocialAppCredentials)
+class SocialAppCredentialsAdmin(admin.ModelAdmin):
+    list_display = ("platform", "client_id", "updated_at")
+    readonly_fields = ("encrypted_client_secret", "created_at", "updated_at")
+
+
 @admin.register(SocialAccount)
 class SocialAccountAdmin(admin.ModelAdmin):
     list_display = ("label", "platform", "is_active", "external_id")