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

packages/db/src/migrations/meta/_journal.json

@@ -288,6 +288,13 @@
       "when": 1784036519678,
       "tag": "0040_loving_spot",
       "breakpoints": true
+    },
+    {
+      "idx": 41,
+      "version": "7",
+      "when": 1784300027960,
+      "tag": "0041_perfect_dexter_bennett",
+      "breakpoints": true
     }
   ]
 }