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).

start9/0.4/startos/actions/toggleSignups.ts

@@ -0,0 +1,118 @@
+import { sdk } from '../sdk'
+
+/**
+ * toggle-signups — StartOS Package Action.
+ *
+ * Sets `InstanceSettings.signupsOpen` (the multi-user signup gate).
+ * When `true`, anyone with the URL can create an account from the app's
+ * /auth/signup page. New users start with no admin privileges and the
+ * full curated exercise library.
+ *
+ * The same toggle is also available in-app at Settings -> Instance
+ * Settings (admin only). Both write to the same singleton row, so
+ * either path works. This StartOS action is the safety hatch for
+ * operators who don't have a working admin login (or aren't logged in
+ * yet on first install).
+ *
+ * Design notes:
+ *   - allowedStatuses: 'only-running'. The app must be running for the
+ *     write to be visible without restart, and the subcontainer needs
+ *     /data mounted writable. We don't require a stop because the
+ *     UPDATE is a single-row write that can't conflict with the
+ *     long-running Next.js server.
+ *   - Single explicit boolean input. Avoids the "I clicked it but did it
+ *     turn on or off?" ambiguity of toggle-style actions.
+ *   - The action does NOT report the current state in `getInput`. It's a
+ *     setter, not a viewer; the in-app Settings page is the dashboard.
+ */
+
+export const toggleSignups = sdk.Action.withInput(
+  'toggle-signups',
+  async () => ({
+    name: 'Set new signups',
+    description:
+      'Allow or disallow anyone with the URL to create a Proof of Work account on this instance. The same toggle exists at in-app Settings -> Instance Settings (admin only).',
+    warning:
+      'When sign-ups are open, anyone who can reach the URL can create an account. Make sure the instance is on a network you trust (LAN, Tor, VPN) before enabling.',
+    visibility: 'enabled',
+    allowedStatuses: 'only-running',
+    group: null,
+  }),
+  sdk.InputSpec.of({
+    signupsOpen: sdk.Value.toggle({
+      name: 'Allow new signups',
+      description: 'On = anyone with the URL can register. Off = closed.',
+      default: false,
+    }),
+  }),
+  async () => null,
+  async ({ effects, input }) => {
+    const flag = input.signupsOpen ? 1 : 0
+
+    await sdk.SubContainer.withTemp(
+      effects,
+      { imageId: 'main' },
+      sdk.Mounts.of().mountVolume({
+        volumeId: 'main',
+        subpath: null,
+        mountpoint: '/data',
+        readonly: false,
+      }),
+      'toggle-signups',
+      async (sc) => {
+        // Defensive: make sure the table exists. The boot-time compat ALTERs
+        // create it, but if this action runs before a first proper boot we
+        // want it to still succeed.
+        const sql = [
+          `CREATE TABLE IF NOT EXISTS InstanceSettings (`,
+          `  id INTEGER PRIMARY KEY DEFAULT 1,`,
+          `  signupsOpen INTEGER NOT NULL DEFAULT 0,`,
+          `  updatedAt DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP`,
+          `);`,
+          `INSERT OR IGNORE INTO InstanceSettings (id, signupsOpen) VALUES (1, ${flag});`,
+          `UPDATE InstanceSettings SET signupsOpen = ${flag}, updatedAt = CURRENT_TIMESTAMP WHERE id = 1;`,
+          `SELECT signupsOpen FROM InstanceSettings WHERE id = 1;`,
+        ].join('\n')
+
+        const res = await sc.execFail(
+          ['sqlite3', '/data/app.db'],
+          { input: sql },
+          30_000,
+        )
+        const observed = res.stdout
+          .toString()
+          .split('\n')
+          .map((s) => s.trim())
+          .filter(Boolean)
+          .pop()
+        if (observed !== String(flag)) {
+          throw new Error(
+            `Aborting: wrote signupsOpen=${flag} but read back ${observed}. /data/app.db may be corrupt.`,
+          )
+        }
+      },
+    )
+
+    return {
+      version: '1',
+      title: input.signupsOpen ? 'Sign-ups enabled' : 'Sign-ups disabled',
+      message: input.signupsOpen
+        ? 'New visitors can now create accounts at /auth/signup.'
+        : 'New sign-ups are now closed. Existing users can still sign in.',
+      result: {
+        type: 'group',
+        value: [
+          {
+            type: 'single',
+            name: 'signupsOpen',
+            description: 'Current value of InstanceSettings.signupsOpen',
+            value: String(input.signupsOpen),
+            copyable: false,
+            qr: false,
+            masked: false,
+          },
+        ],
+      },
+    }
+  },
+)