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.

lib/validations.ts

@@ -666,7 +666,10 @@ export const linkSubmissionsSchema = z.object({
   email: emailField(),
 })
 
-export const getPublicFormSchema = z.object({ slug: formSlug })
+export const getPublicFormSchema = z.object({
+  space: z.string().min(1).max(100),
+  slug: formSlug,
+})
 
 // ─── Generic ID schemas ──────────────────────────────────────────────────────