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/new/page.tsx

@@ -5,7 +5,7 @@ import { FormBuilder } from '@/components/forms/form-builder'
 export const dynamic = 'force-dynamic'
 
 export default async function NewFormPage() {
-  await requireFormsManagerPage()
+  const { spaceSlug } = await requireFormsManagerPage()
 
   return (
     <>
@@ -15,6 +15,7 @@ export default async function NewFormPage() {
       <div className="p-4 md:p-6">
         <FormBuilder
           initial={{
+            spaceSlug,
             slug: '',
             title: '',
             description: '',