Visibility phase 1: add site_members value + 4-option dropdown

First step of the public-viewing feature (design: docs/design/tree-visibility.md).
No non-member behavior change yet — this only widens the vocabulary and UI.

- TreeVisibility gains `site_members` (any authenticated user of the instance),
  giving the four-level model: public / site_members / unlisted / private.
- Alembic migration adds the enum value via an autocommit block (ALTER TYPE
  ADD VALUE can't run in a transaction on older Postgres); downgrade is a no-op
  since PG can't drop an enum value.
- Regenerated openapi.json + frontend TS client.
- Trees-list dropdown now offers Private / Public – Members / Unlisted / Public
  with an explanatory tooltip.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Signed-off-by: Justin Paul <justin@jpaul.me>

backend/app/models/enums.py

@@ -9,9 +9,10 @@ import enum
 
 
 class TreeVisibility(enum.StrEnum):
-    public = "public"
-    unlisted = "unlisted"
-    private = "private"
+    public = "public"  # anyone on the web (anonymous), listed + search-indexable
+    site_members = "site_members"  # any authenticated user of this instance
+    unlisted = "unlisted"  # anyone with the link (anonymous), not listed/indexed
+    private = "private"  # members only (default)
 
 
 class MembershipRole(enum.StrEnum):