Fix four deployability blockers found by QA (closes #189, closes #190, closes #191, closes #192)

- CSRF over plain HTTP (#189): gorilla/csrf assumed https for its
  same-origin check, so setup and every POST returned 403 over plain
  HTTP. Gate csrf.PlaintextHTTPRequest on a new UPAAS_PLAINTEXT_HTTP
  config value; the default keeps https, correct for a TLS-terminating
  reverse proxy. The README plain-HTTP recipe now sets it.
- git image never pulled (#190): ensureImage pulls alpine/git (pinned
  digest unchanged) when absent, before the clone container is created.
- port-mapping 500 (#192): the ports delete form used {{ .CSRFField }}
  inside {{range .Ports}}, where the dot is a *models.Port; use
  {{ $.CSRFField }} like the labels and volumes blocks.
- env-var 403 (#191): the editor read the CSRF token from $el (the
  submitting form, which has none) instead of $root, sending an empty
  token; read from $root.

Model: opus-4-8

internal/config/config.go

@@ -49,6 +49,7 @@ type Config struct {
 	DockerHost      string
 	SentryDSN       string
 	MaintenanceMode bool
+	PlaintextHTTP   bool // clients reach µPaaS over plain HTTP (no TLS-terminating proxy)
 	MetricsUsername string
 	MetricsPassword string
 	SessionSecret   string `json:"-"`
@@ -100,6 +101,7 @@ func setupViper(name string) {
 	viper.SetDefault("DOCKER_HOST", "unix:///var/run/docker.sock")
 	viper.SetDefault("SENTRY_DSN", "")
 	viper.SetDefault("MAINTENANCE_MODE", false)
+	viper.SetDefault("PLAINTEXT_HTTP", false)
 	viper.SetDefault("METRICS_USERNAME", "")
 	viper.SetDefault("METRICS_PASSWORD", "")
 	viper.SetDefault("SESSION_SECRET", "")
@@ -135,6 +137,7 @@ func buildConfig(log *slog.Logger, params *Params) (*Config, error) {
 		DockerHost:      viper.GetString("DOCKER_HOST"),
 		SentryDSN:       viper.GetString("SENTRY_DSN"),
 		MaintenanceMode: viper.GetBool("MAINTENANCE_MODE"),
+		PlaintextHTTP:   viper.GetBool("PLAINTEXT_HTTP"),
 		MetricsUsername: viper.GetString("METRICS_USERNAME"),
 		MetricsPassword: viper.GetString("METRICS_PASSWORD"),
 		SessionSecret:   viper.GetString("SESSION_SECRET"),