Add detailed skip tracking for phone imports

- Track skipped_no_phone and skipped_no_id separately
- Display skip information in admin UI with warning icon
- Clarify that empty phone numbers cannot be imported (PK constraint)
- Update documentation to explain expected skip behavior
- Example: 143 rows without phone numbers is correct, not an error

When importing PHONE.csv with empty phone numbers:
- Rows are properly skipped (cannot have NULL in primary key)
- User sees: '⚠️ Skipped: 143 rows without phone number'
- This is expected behavior, not a bug

app/templates/admin.html

@@ -179,6 +179,8 @@
                                                 {% endfor %}
                                             </ul>
                                         </details>
+                                        {% elif item.skip_info %}
+                                        <small class="text-warning">⚠️ Skipped: {{ item.skip_info }}</small>
                                         {% else %}
                                         <span class="text-muted">None</span>
                                         {% endif %}