v0.88.0: app-email SMTP relay (in-process shim + per-app injection)

In-process go-smtp shim (Shape 1): apps → shim → hub → Resend, Resend key stays
hub-side. From-header allowlist (reject 5xx pre-hub), single-shot raw-MIME forward,
status→SMTP mapping. Global + per-app toggles gate compose-time env injection from
.felhom.yml smtp_mapping. Hungarian UI on settings + app config pages.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

controller/internal/stacks/manager.go

@@ -883,6 +883,15 @@ func (m *Manager) stackEnv(stackDir string) []string {
 		env = withUserdataPath(env, appCfg.Env["HDD_PATH"])
 	}
 
+	// App-email relay env (appended LAST so it wins over any app.yaml default). Returns nil unless
+	// global + per-app toggles are on and the app declares an smtp_mapping — so apps with email off
+	// get nothing injected. Derived, never persisted to app.yaml.
+	emailOn := appCfg != nil && appCfg.EmailEnabled
+	meta := LoadMetadata(stackDir)
+	if smtp := m.smtpEnv(&meta, emailOn); len(smtp) > 0 {
+		env = append(env, smtp...)
+	}
+
 	return env
 }