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/page.tsx

@@ -4,7 +4,7 @@ import { FormsClient } from './forms-client'
 export const dynamic = 'force-dynamic'
 
 export default async function FormsPage() {
-  const { supabase, member } = await requireFormsManagerPage()
+  const { supabase, member, spaceSlug } = await requireFormsManagerPage()
 
   const { data: forms } = await supabase
     .from('forms')
@@ -12,5 +12,5 @@ export default async function FormsPage() {
     .eq('space_id', member.space_id)
     .order('created_at', { ascending: false })
 
-  return <FormsClient forms={forms ?? []} />
+  return <FormsClient forms={forms ?? []} spaceSlug={spaceSlug} />
 }