Multi-user: self-serve sign-up gated by admin-toggleable flag

Schema
- User.isAdmin: Boolean default false (Prisma)
- New InstanceSettings singleton (id=1) holding signupsOpen flag

Boot-time compat ALTERs (docker_entrypoint.sh)
- Adds User.isAdmin column to legacy snapshots; auto-promotes the oldest
  user to admin if no admin exists yet, so workout-log -> proof-of-work
  cutover preserves admin functionality with no manual SQL.
- Creates InstanceSettings table + singleton row (signupsOpen=0) for any
  snapshot that doesn't have it.

App: sign-up flow
- /auth/signup page: server component that reads InstanceSettings
  upfront. If sign-ups are closed it shows a closed-instance message and
  a back-to-sign-in link rather than a dead form. If open it renders
  SignupForm (client) which calls signupAction (server).
- signupAction: re-checks the flag (defense in depth), validates email
  format / 8-char password / matching confirm, blocks duplicate-email
  enumeration with a generic error, creates the user with isAdmin=false,
  seeds default UserPreferences, ensures the curated exercise library
  for the new user (lib/library.ts upserts every entry), then issues a
  session cookie.
- Login page now links to /auth/signup; old "Demo: admin@example.com /
  password" footer (which was wrong anyway) removed.

App: admin in-app toggle
- Settings page renders new AdminInstanceSettings component for admins
  only. Optimistic toggle posts to /api/admin/signups; error rollback
  on failure.
- /api/admin/signups: GET returns current flag (any authed user, so the
  UI knows whether to show the sign-up CTA later); POST flips it
  (admin only).

StartOS package action
- toggle-signups: same setter as the in-app toggle, accessible from the
  StartOS UI without an admin login. Single boolean input. Asserts the
  read-back value matches what was written before reporting success.
- changeAdminCredentials now keys the UPDATE on
  `WHERE isAdmin = 1 ORDER BY createdAt ASC LIMIT 1` (was: just
  ORDER BY createdAt) — correct under multi-user.

Release notes / docs
- v1.0.0:1 release notes expanded to call out multi-user as part of
  the cutover release (no separate version needed since this is the
  first proof-of-work release shipping to anyone).
- Root README: short Multi-user section explaining both toggle paths
  and that new users get the curated library automatically.
- README dev setup adds `npx prisma generate` step (required after
  schema changes for local dev).

README.md

@@ -20,11 +20,27 @@ Everything else is generated at build time.
 ```sh
 cd proof-of-work
 npm install
+npx prisma generate       # important after schema changes
 npx prisma db push        # create the dev DB at prisma/data/app.db
-npm run db:seed           # admin@local / workout123 + curated exercise library
+npm run db:seed           # admin@local / workout123 + curated library + admin flag
 npm run dev               # http://localhost:3000
 ```
 
+## Multi-user
+
+Every install starts with one admin user (`admin@local`) and **sign-ups
+closed**. To open the instance to additional users:
+
+- In-app: log in as admin -> **Settings -> Instance Settings ->
+  Allow new sign-ups**.
+- StartOS: **Services -> Proof of Work -> Actions -> Set new signups**.
+
+Both write to the same `InstanceSettings` row; either path works.
+
+When sign-ups are open, anyone reaching the URL can create an account at
+`/auth/signup`. New users start with no admin privileges and are
+automatically seeded the full curated exercise library.
+
 ## Building the StartOS package
 
 See **[start9/0.4/DEPLOY_040.md](start9/0.4/DEPLOY_040.md)** for the full