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/app/(app)/explore/page.tsx

@@ -28,7 +28,7 @@ export type RecipeResult = {
   baseServings: number;
   prepMins: number | null;
   cookMins: number | null;
-  visibility: "private" | "unlisted" | "public";
+  visibility: "private" | "unlisted" | "public" | "followers";
   tags: string[];
   isBatchCook: boolean;
   sourceUrl: string | null;