Add bio field and daily streak check-in for gamification

- User.Bio: a short (300 char) freeform profile bio, editable via
  PATCH /profile/bio, shown on discover cards and candidate detail (richer
  profiles beyond the fixed onboarding fields).
- User.StreakCount/LastActiveDate: POST /profile/checkin increments the
  streak on consecutive days, resets after a gap, no-ops if already
  checked in today. Backs a daily-return hook on the frontend.

internal/handler/common.go

@@ -43,6 +43,8 @@ func toUserProfile(u *models.User) dto.UserProfile {
 		State:           string(u.State),
 		BlockedReason:   u.BlockedReason,
 		BlockedAt:       u.BlockedAt,
+		Bio:             u.Bio,
+		StreakCount:     u.StreakCount,
 	}
 	if u.Gender != nil {
 		g := string(*u.Gender)