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/messages/en.json

@@ -223,9 +223,11 @@
     "visibilityMenuLabel": "Visibility",
     "makePublic": "Make public",
     "makeUnlisted": "Make unlisted",
+    "makeFollowersOnly": "Make followers only",
     "makePrivate": "Make private",
     "visibility": {
       "private": "Private",
+      "followers": "Followers only",
       "unlisted": "Unlisted",
       "public": "Public"
     },