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/dto/auth_dto.go

@@ -49,4 +49,6 @@ type UserProfile struct {
 	State           string     `json:"state"`
 	BlockedReason   *string    `json:"blocked_reason,omitempty"`
 	BlockedAt       *time.Time `json:"blocked_at,omitempty"`
+	Bio             *string    `json:"bio,omitempty"`
+	StreakCount     int        `json:"streak_count"`
 }