fix(backend): read a tooth code whatever script its digits are in

The extraction model transcribes Persian speech, so it can hand back "۲۶"
in Persian digits or "2 6" from a digit-by-digit dictation. Both were
compared literally against /^[1-8][1-8]$/, missed, and fell through to the
positional branch with no quadrant — where the tooth was reported as "not
understood". The clinician loses a tooth and is told the words were the
problem.

normalizeFdiCode() now runs at both the branch choice and the final
validation, so the two cannot disagree. toLatinDigits moves out of
jalali.ts into common/digits.ts: it was exported but unused in production,
and a tooth module reaching into the calendar module would read as an
accident.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

backend/src/modules/voice/tooth-intent.resolver.spec.ts

@@ -134,6 +134,14 @@ describe('resolveToothIntents', () => {
     }
   });
 
+  it('reads a spoken number as its FDI code, digits in any script', () => {
+    // The product rule: the number the clinician says IS the tooth. 26 = quadrant 2
+    // (patient's upper left) + position 6 = first molar.
+    for (const raw of ['26', ' 26 ', '2 6', '\u06F2\u06F6', '\u0662\u0666']) {
+      expect(resolveToothIntents([explicit(raw, 'x')]).teeth).toEqual(['26']);
+    }
+  });
+
   it('trims an explicit code, matching normalizeTeeth', () => {
     expect(resolveToothIntents([explicit(' 14 ', 'x')]).teeth).toEqual(['14']);
   });