feat(organizations): email inviter when membership invite is accepted

When a user accepts an org invite, send a notification email to the
person who sent the invite with membership details (member name, email,
org name, role) and an optional View Organization button.

Added build_invite_accepted_html() template to email_templates.py,
wired it into the accept_invite() handler, and added a test case.

test_email.py

@@ -148,8 +148,28 @@ def test_html_email():
         success = provider.send(message)
         print(f"Result: {'✅ SUCCESS' if success else '❌ FAILED'}")
 
+        # Test 8: Invite Accepted
+        print("\n--- Test 8: Invite Accepted ---")
+        html_body = email_templates.build_invite_accepted_html(
+            inviter_name="Admin User",
+            member_name="New Member",
+            member_email="newmember@example.com",
+            org_name="Acme Corporation",
+            role="Member",
+            org_link="https://secuird.tech/organizations/org-123",
+        )
+        message = EmailMessage(
+            to="cory@hawkvelt.id.au",
+            subject="New Member accepted your invitation to Acme Corporation",
+            body="Plain text version: New Member has accepted your invitation.",
+            html_body=html_body,
+            from_address="Secuird <noreply@secuird.tech>",
+        )
+        success = provider.send(message)
+        print(f"Result: {'✅ SUCCESS' if success else '❌ FAILED'}")
+
         print("\n" + "=" * 50)
-        print("All 7 email templates sent!")
+        print("All 8 email templates sent!")
         print("=" * 50)