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/docker_entrypoint.sh

@@ -75,6 +75,35 @@ if command -v sqlite3 >/dev/null 2>&1 && [ -f "$DB_PATH" ]; then
     log "adding missing column Workout.deletedAt"
     sqlite3 "$DB_PATH" "ALTER TABLE Workout ADD COLUMN deletedAt DATETIME;"
   fi
+
+  # Multi-user support shipped in v1.0.0:1: User.isAdmin column +
+  # InstanceSettings singleton. New install: seed.ts creates both. Upgrade
+  # from a snapshot pulled off the legacy `workout-log` package: this block
+  # adds them in place, then promotes the oldest user to admin so the
+  # in-app admin Settings panel + change-credentials action keep working.
+  if ! sqlite3 "$DB_PATH" "PRAGMA table_info('User');" 2>/dev/null | grep -q "|isAdmin|"; then
+    log "adding missing column User.isAdmin (default 0)"
+    sqlite3 "$DB_PATH" "ALTER TABLE User ADD COLUMN isAdmin INTEGER NOT NULL DEFAULT 0;"
+    log "promoting oldest user to admin (one-shot, only if no admin exists)"
+    sqlite3 "$DB_PATH" \
+      "UPDATE User SET isAdmin = 1 \
+       WHERE id = (SELECT id FROM User ORDER BY createdAt ASC LIMIT 1) \
+         AND NOT EXISTS (SELECT 1 FROM User WHERE isAdmin = 1);"
+  fi
+
+  if ! sqlite3 "$DB_PATH" \
+    "SELECT name FROM sqlite_master WHERE type='table' AND name='InstanceSettings';" \
+    2>/dev/null | grep -q InstanceSettings; then
+    log "creating InstanceSettings table + singleton row (signupsOpen=0)"
+    sqlite3 "$DB_PATH" \
+      "CREATE TABLE InstanceSettings ( \
+         id INTEGER PRIMARY KEY DEFAULT 1, \
+         signupsOpen INTEGER NOT NULL DEFAULT 0, \
+         updatedAt DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP \
+       );"
+    sqlite3 "$DB_PATH" \
+      "INSERT OR IGNORE INTO InstanceSettings (id, signupsOpen) VALUES (1, 0);"
+  fi
 fi
 
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