feat(admin): Paddle client behind an interface, config, and the event idempotency record

Client is a thin REST client (net/http) rather than the vendor SDK: the surface
we need is two calls, and a hand-rolled client has no version-drift risk and no
dependency in go.sum. All Paddle wire shapes live only in http.go.

PADDLE_API_KEY and PADDLE_WEBHOOK_SECRET are boot-required — an unverified
webhook endpoint is one anyone can issue licences through. paddle_events carries
a unique index on event_id for webhook idempotency.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

admin/cmd/main.go

@@ -21,6 +21,7 @@ import (
 	"github.com/mrhid6/vantage/admin/internal/lifecycle"
 	"github.com/mrhid6/vantage/admin/internal/mail"
 	"github.com/mrhid6/vantage/admin/internal/models"
+	"github.com/mrhid6/vantage/admin/internal/paddle"
 )
 
 func main() {
@@ -34,6 +35,10 @@ func main() {
 	licensing.SetSigningKey(cfg.SigningKey)
 	api.SetAppLoginURL(cfg.AppLoginURL)
 
+	if _, err := paddle.Init(cfg.PaddleAPIKey, cfg.PaddleEnv); err != nil {
+		log.Fatalf("paddle init: %v", err)
+	}
+
 	mail.Init(mail.Config{
 		Host: cfg.SMTPHost, Port: cfg.SMTPPort, From: cfg.SMTPFrom,
 		Username: cfg.SMTPUsername, Password: cfg.SMTPPassword,