Fix avatar misgendering, returning caller overflow, false callbacks

- Avatar prefetch checks gender marker, re-fetches on mismatch
- Returning callers need 2+ actual calls before re-eligible (was 1)
- Promotion rate lowered 10% → 5% to prevent pool flooding
- Callback injection skipped for returning callers (already have context)
- Show history clarifies "you are NOT that caller" to prevent identity confusion

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/services/regulars.py

@@ -44,7 +44,7 @@ class RegularCallerService:
         import random
         if not self._regulars:
             return []
-        available = [r for r in self._regulars if len(r.get("call_history", [])) > 0]
+        available = [r for r in self._regulars if len(r.get("call_history", [])) > 1]
         if not available:
             return []
         return random.sample(available, min(count, len(available)))