feat(post-form): inject active-trip parent server-side (U1)

Derive the entry write target from site.active_trip at submit time in
cache-on-save's onFormValidationProcessed handler, and fail closed
(ValidationException) when no active trip is set. Removes the hardcoded
pageconfig.parent that had to be hand-synced with active_trip.

Refs R1, R2, AE2, KTD1.

pages/02.post/post-form.md

@@ -4,9 +4,9 @@ template: post-form
 access:
     site.login: true
 
-# Keep in sync with active_trip in user/config/site.yaml
+# Parent (write target) is injected server-side from site.active_trip by the
+# cache-on-save plugin (onFormValidationProcessed) — no manual sync needed.
 pageconfig:
-    parent: '/trips/italy-2026-demo/dailies'
     slug_field: 'date,title'
     overwrite_mode: false