Remove manual course creation; import-only with per-tee stroke indices

Courses can now only be added via Golf Course API import, which
provides the full data model: per-tee slope/rating/gender/SI
(stroke_indices JSON) and front/back split ratings. Manual creation
required admins to enter SI by hand and didn't match the API model.

Changes:
- Remove GET/POST /admin/courses/new routes and new_course.html
- Remove _parse_hole_form helper (no longer needed)
- Edit course: par-only editing; stroke_index preserved from import,
  not exposed or overwritten in the admin form
- courses.html: replace "+ New course" with "+ Import course" button
- course_search.html: remove "+ Manual" shortcut
- edit_course.html: remove SI column from hole grid

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

app/templates/admin/course_search.html

@@ -15,7 +15,6 @@
 
   <div style="display:flex; gap:0.5rem; justify-content:flex-end; margin-bottom:1rem;">
     <a href="/admin/courses" class="btn" style="width:auto; padding:0.5rem 1rem; background:var(--surface2); color:var(--text);">← Courses</a>
-    <a href="/admin/courses/new" class="btn btn-primary" style="width:auto; padding:0.5rem 1rem;">+ Manual</a>
   </div>
 
   {% if not api_configured %}