feat: replace UUID v4 with UUID v7 across backend

Define uuid_generate_v7() in the first migration and switch all table
primary key defaults to it. Remove the uuid-ossp extension dependency.
Update refresh token and request ID generation in Go code to use
uuid.NewV7() from the existing google/uuid v1.6.0 library.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/internal/auth/jwt.go

@@ -42,7 +42,7 @@ func (j *JWTManager) GenerateAccessToken(userID, plan string) (string, error) {
 }
 
 func (j *JWTManager) GenerateRefreshToken() (string, time.Time) {
-	token := uuid.NewString()
+	token := uuid.Must(uuid.NewV7()).String()
 	expiresAt := time.Now().Add(j.refreshDuration)
 	return token, expiresAt
 }