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>

frontend/app/trees/page.tsx

@@ -95,10 +95,17 @@ export default function TreesPage() {
                       setVisibility(tree.id, e.target.value as NonNullable<Tree["visibility"]>)
                     }
                     aria-label="Tree visibility"
-                    title="Who can see this tree. Living people stay protected even when public."
+                    title={
+                      "Who can see this tree (living people stay protected regardless):\n" +
+                      "• Private — only you and people you invite\n" +
+                      "• Members — any signed-in user on this site\n" +
+                      "• Unlisted — anyone with the link (not listed or indexed)\n" +
+                      "• Public — anyone on the web; listed and search-indexable"
+                    }
                     className="rounded-md border border-[var(--border)] bg-[var(--surface)] px-2 py-1 text-xs uppercase tracking-wide text-bronze focus-visible:border-bronze focus-visible:outline-none"
                   >
                     <option value="private">Private</option>
+                    <option value="site_members">Public – Members</option>
                     <option value="unlisted">Unlisted</option>
                     <option value="public">Public</option>
                   </select>