fix: return patient national code from profile in patients list

National code is stored on the profiles table, not users, so the patients
search returned null user_national_code for patients that actually have one
— which blocked selecting an existing patient on the appointment create
form. Backfill user_national_code from the profile (batch query) in the
list endpoint. On the create page, show a picked patient's stored national
code read-only and only prompt for input when the record genuinely lacks
one. Add backend tests and update docs/api/patient.md.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

assets/admin/pages/AppointmentCreatePage.tsx

@@ -217,11 +217,20 @@ export default function AppointmentCreatePage() {
                   <button type="button" className="btn sm ghost"
                     onClick={() => { setPicked(null); setNationalCode(''); }}>تغییر</button>
                 </div>
-                <label style={label}>کد ملی</label>
-                <div className="field" style={{ marginTop: 6, height: 44 }}>
-                  <DigitInput value={nationalCode} onChange={setNationalCode} maxDigits={10}
-                    placeholder="کد ملی مراجعه کننده" />
-                </div>
+                {/^\d{10}$/.test((picked.user_national_code ?? '').replace(/\D/g, '')) ? (
+                  <div style={{ display: 'flex', alignItems: 'center', justifyContent: 'space-between', fontSize: 13 }}>
+                    <span style={{ color: 'var(--text-3)' }}>کد ملی</span>
+                    <span dir="ltr" style={{ fontWeight: 600 }}>{nationalCode}</span>
+                  </div>
+                ) : (
+                  <>
+                    <label style={label}>کد ملی (ثبت‌نشده — وارد کنید)</label>
+                    <div className="field" style={{ marginTop: 6, height: 44 }}>
+                      <DigitInput value={nationalCode} onChange={setNationalCode} maxDigits={10}
+                        placeholder="کد ملی مراجعه کننده" />
+                    </div>
+                  </>
+                )}
               </div>
             )}
           </>