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/edit_course.html

@@ -58,12 +58,6 @@
             <option value="4" {% if h.par == 4 %}selected{% endif %}>4</option>
             <option value="5" {% if h.par == 5 %}selected{% endif %}>5</option>
           </select>
-          <select name="si_{{ h.hole_number }}" class="hole-par-select">
-            <option value="">—</option>
-            {% for s in range(1, course.holes + 1) %}
-            <option value="{{ s }}" {% if h.stroke_index == s %}selected{% endif %}>{{ s }}</option>
-            {% endfor %}
-          </select>
         </div>
         {% endfor %}
       </div>