feat: followers-only recipe visibility

Adds a fourth visibility tier alongside private/unlisted/public:
"followers" — visible to the author and anyone who follows them,
excluded from public search/explore/profile listings and from the
anonymous /r/[id] share route, included in the Following feed. The
in-app recipe detail gate and the public share route both check
follow status directly against the DB rather than the union type
alone, since neither previously had any per-viewer access logic for
a non-public/non-owner case.

Requires the generated migration (0041) to run against a live DB —
not applied in this sandbox (no Docker here); run `pnpm db:migrate`.

v0.41.0

apps/web/components/recipe/share-recipe-button.tsx

@@ -17,7 +17,7 @@ export function ShareRecipeButton({ recipeId, visibility }: { recipeId: string;
       toast.error(t("shareCopyFailed"));
       return;
     }
-    if (visibility === "private") {
+    if (visibility === "private" || visibility === "followers") {
       toast.info(t("shareNotPublicNotice"));
     } else {
       toast.success(t("shareLinkCopied"));