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/middleware/middleware.go

@@ -255,12 +255,34 @@ func (m *Middleware) SessionAuth() func(http.Handler) http.Handler {
 }
 
 // CSRF returns CSRF protection middleware using gorilla/csrf.
+//
+// gorilla/csrf assumes the request scheme is https for its same-origin check
+// unless the request is marked plaintext. A TLS-terminating reverse proxy
+// (the default deployment) presents https to the browser, so the default is
+// correct there. When µPaaS is reached over plain HTTP — directly, or behind a
+// proxy that does not terminate TLS — set UPAAS_PLAINTEXT_HTTP so the origin
+// check compares against http:// and setup over plain HTTP works.
 func (m *Middleware) CSRF() func(http.Handler) http.Handler {
-	return csrf.Protect(
+	protect := csrf.Protect(
 		[]byte(m.params.Config.SessionSecret),
-		csrf.Secure(false), // Allow HTTP for development; reverse proxy handles TLS
+		csrf.Secure(false), // cookie Secure flag; TLS is terminated upstream
 		csrf.Path("/"),
 	)
+
+	if !m.params.Config.PlaintextHTTP {
+		return protect
+	}
+
+	return func(next http.Handler) http.Handler {
+		protected := protect(next)
+
+		return http.HandlerFunc(func(
+			writer http.ResponseWriter,
+			request *http.Request,
+		) {
+			protected.ServeHTTP(writer, csrf.PlaintextHTTPRequest(request))
+		})
+	}
 }
 
 // loginRateLimit configures the login rate limiter.