Add HTML email templates for magic link sign-in

Extract inline email body into separate Jinja2 templates
(_email/magic_link.html and .txt) with a styled sign-in
button and fallback plain-text link.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

templates/_email/magic_link.txt

@@ -0,0 +1,8 @@
+Hello,
+
+Click this link to sign in:
+{{ link_url }}
+
+This link will expire in 15 minutes.
+
+If you did not request this, you can ignore this email.