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/social-follows.ts

@@ -0,0 +1,15 @@
+import { db, userFollows, and, eq } from "@epicure/db";
+
+/** True if `followerId` follows `followingId`. Used to gate "followers only"
+ * visibility content — the same leftJoin+isNotNull idiom used elsewhere
+ * (e.g. the for-you feed's private-author check) collapsed into a single
+ * existence check for the single-recipe case. */
+export async function isFollowing(followerId: string, followingId: string): Promise<boolean> {
+  if (followerId === followingId) return true;
+  const [row] = await db
+    .select({ followerId: userFollows.followerId })
+    .from(userFollows)
+    .where(and(eq(userFollows.followerId, followerId), eq(userFollows.followingId, followingId)))
+    .limit(1);
+  return !!row;
+}