feat(forms): space-scoped public URL /f/[space]/[slug] (P6)

- migration 028: drop global UNIQUE(forms.slug), add
  UNIQUE(space_id, slug); schema.sql mirrored. Form slugs are now
  unique per space (fixes cross-tenant slug squatting)
- getPublicForm now takes { space, slug }: resolves the space by its
  slug then the form by space_id + slug (maybeSingle)
- route moved app/f/[slug] -> app/f/[space]/[slug]; the public client
  submits by formId (slug is no longer globally unambiguous) and the
  sign-in link round-trips the full /f/space/slug path
- forms-guard returns the space slug; the builder + forms list show
  the full /f/<space>/<slug> share URL; new/edit pass it through
- middleware /f allowlist already covers /f/<space>/<slug>
- getPublicFormSchema + tests updated (333 pass)

build verified (Compiled successfully, exit 0; new route present). No deploy.

app/(app)/forms/[id]/edit/page.tsx

@@ -8,7 +8,7 @@ export const dynamic = 'force-dynamic'
 
 export default async function EditFormPage({ params }: { params: Promise<{ id: string }> }) {
   const { id } = await params
-  const { supabase, member } = await requireFormsManagerPage()
+  const { supabase, member, spaceSlug } = await requireFormsManagerPage()
 
   const { data: form } = await supabase
     .from('forms')
@@ -28,6 +28,7 @@ export default async function EditFormPage({ params }: { params: Promise<{ id: s
         <FormBuilder
           initial={{
             id: form.id,
+            spaceSlug,
             slug: form.slug,
             title: form.title,
             description: form.description ?? '',