feat(admin): cloudprov, the instance provisioning write path

Admin's second and final write path into the control plane. It creates
instances and users and nothing else; inject still owns exactly three
licence fields and is untouched.

The owner's password hash is copied from the HQ account, not shared. The
two diverge on the next password change, which is accepted: propagating a
hash across two databases is worse than two passwords that started equal.

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

admin/internal/config/config.go

@@ -22,6 +22,7 @@ type Config struct {
 	RedisPassword   string
 	SigningKey      string
 	PublicURL       string
+	AppLoginURL     string
 	AllowedOrigins  []string
 	TrustProxy      bool
 	Addr            string
@@ -63,6 +64,7 @@ func Load() (Config, error) {
 		RedisPassword: os.Getenv("REDIS_PASSWORD"),
 		SigningKey:    os.Getenv("LICENSE_SIGNING_KEY"),
 		PublicURL:     strings.TrimSuffix(os.Getenv("PUBLIC_URL"), "/"),
+		AppLoginURL:   os.Getenv("APP_LOGIN_URL"),
 		TrustProxy:    strings.EqualFold(os.Getenv("TRUST_PROXY"), "true"),
 		Addr:          ":" + envOr("PORT", "8083"),