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/lib/visibility.ts

@@ -0,0 +1,19 @@
+import { recipes, userFollows, eq, or, and, inArray, sql } from "@epicure/db";
+
+/**
+ * Drizzle condition: true when `viewerId` may view a given recipe row — the
+ * author, always; `public`/`unlisted` recipes, to anyone; `followers`-only
+ * recipes, only to viewers who follow the author. Meant to be `and()`ed with
+ * an `eq(recipes.id, id)` (or similar) in a single query, so the DB does the
+ * filtering rather than fetching content the viewer can't see.
+ */
+export function visibleToViewer(viewerId: string) {
+  return or(
+    eq(recipes.authorId, viewerId),
+    inArray(recipes.visibility, ["public", "unlisted"]),
+    and(
+      eq(recipes.visibility, "followers"),
+      sql`exists (select 1 from ${userFollows} where ${userFollows.followerId} = ${viewerId} and ${userFollows.followingId} = ${recipes.authorId})`
+    )
+  );
+}