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/internal/config/config.go

@@ -29,6 +29,10 @@ type Config struct {
 	Addr            string
 	ReapAfter       time.Duration
 
+	PaddleEnv           string // "sandbox" or "production"
+	PaddleAPIKey        string
+	PaddleWebhookSecret string
+
 	SMTPHost     string
 	SMTPPort     string
 	SMTPFrom     string
@@ -75,6 +79,10 @@ func Load() (Config, error) {
 		SMTPFrom:     os.Getenv("SMTP_FROM"),
 		SMTPUsername: os.Getenv("SMTP_USERNAME"),
 		SMTPPassword: os.Getenv("SMTP_PASSWORD"),
+
+		PaddleEnv:           envOr("PADDLE_ENV", "sandbox"),
+		PaddleAPIKey:        os.Getenv("PADDLE_API_KEY"),
+		PaddleWebhookSecret: os.Getenv("PADDLE_WEBHOOK_SECRET"),
 	}
 
 	var missing []string
@@ -85,6 +93,10 @@ func Load() (Config, error) {
 		"LICENSE_SIGNING_KEY": c.SigningKey,
 		"PUBLIC_URL":          c.PublicURL,
 		"ADMIN_ORIGIN":        os.Getenv("ADMIN_ORIGIN"),
+		// An unverified webhook endpoint is one anyone can issue licences
+		// through, so the secret and API key are boot-required.
+		"PADDLE_API_KEY":        c.PaddleAPIKey,
+		"PADDLE_WEBHOOK_SECRET": c.PaddleWebhookSecret,
 	} {
 		if v == "" {
 			missing = append(missing, name)