Initial commit: SummerCMS Go planning docs and research

Records the move from Scala to Go, the compiled-plugin decision, the
Płytarium port as the v1 target, and the Go ecosystem research that
backs the choice. No code yet.

.planning/seeds/keios-port-themes-payments.md

@@ -0,0 +1,25 @@
+---
+title: keios.eu port — server-rendered themes and payments
+trigger_condition: Płytarium v1 is running in production on SummerCMS Go (vue-fonoteka-app unchanged).
+planted_date: 2026-09-16
+---
+
+# keios.eu port: themes and payments
+
+Second port target after Płytarium. keios.eu is the smallest Golem15 project that has both a Twig theme and a payment flow.
+
+| Aspect | keios.eu |
+|---|---|
+| Golem15 plugins | apparatus, dualformfield, faq, golem, journal, knobwidget, paymentgateway, pgstripe, quote, sitemap, translate, user, websockets |
+| Theme | `wn-keioseu-theme`, 14 template files |
+| Plugin PHP lines | ~74k |
+
+## What it adds to SummerCMS
+
+- Theme engine with Twig-like syntax (`pongo2` per research, used only for themes) and components dropped into templates.
+- `paymentgateway` and `pgstripe` port. Stripe Go SDK is official.
+- `journal`, `faq`, `quote` as classic content plugins with admin forms.
+
+## Related
+
+- `../notes/v1-target-plytarium.md`