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/discovery_dto.go

@@ -5,6 +5,7 @@ type DiscoverCandidate struct {
 	ID         uint     `json:"id"`
 	FirstName  string   `json:"first_name"`
 	Age        int      `json:"age"`
+	Bio        string   `json:"bio,omitempty"`
 	Photos     []string `json:"photos"`
 	DistanceKM *float64 `json:"distance_km,omitempty"`
 	Interests  []string `json:"interests,omitempty"`
@@ -58,6 +59,7 @@ type CandidateDetail struct {
 	ID                      uint              `json:"id"`
 	FirstName               string            `json:"first_name"`
 	Age                     int               `json:"age"`
+	Bio                     string            `json:"bio,omitempty"`
 	Photos                  []string          `json:"photos"`
 	DistanceKM              *float64          `json:"distance_km,omitempty"`
 	HeightCM                *int              `json:"height_cm,omitempty"`