## Why

CandleAnnotator is currently a single-user application with no authentication — all data (charts, annotations, models, settings) is global. To support multiple users, each with their own isolated workspace, we need user accounts with registration, login, and per-user data isolation. This unlocks the ability to deploy as a multi-tenant SaaS.

## What Changes

- Add user accounts with email/password registration and Google OAuth login
- Add NextAuth.js authentication with JWT sessions
- Add landing page (public, from Lovable design) at `/`
- Add login page at `/login` (from Lovable design)
- Add register page at `/register` (from Lovable design)
- Move the existing app workspace to `/app` (protected route)
- Add `users` table to PostgreSQL schema
- Add `user_id` foreign key to all existing data tables: `charts`, `annotations`, `span_annotations`, `annotation_types`, `span_label_types`
- Filter all API queries by authenticated user's ID
- Add user settings page at `/app/settings` with: change password, display name, email preferences, delete account
- Add Google OAuth provider configuration
- Add auth middleware to protect `/app/*` and `/api/*` routes
- Seed default annotation types per user on registration

## Capabilities

### New Capabilities
- `user-auth`: User registration (email+password), login, Google OAuth, JWT sessions, password hashing, auth middleware, protected routes
- `landing-page`: Public landing page at `/` with Lovable design, nav links to login/register
- `login-page`: Login page at `/login` with email/password form and Google OAuth button (Lovable design)
- `register-page`: Registration page at `/register` with name/email/password form (Lovable design)
- `user-settings`: Settings page at `/app/settings` — change password, update display name, manage email, delete account
- `user-data-isolation`: Per-user data scoping — all existing tables get `user_id` FK, all queries filtered by authenticated user, default data seeded on registration

### Modified Capabilities
- `postgres-data-layer`: Add `users` table, add `user_id` column to all existing tables, update schema and migrations
- `backend-api`: All API routes require authentication, filter queries by `user_id` from session
- `ui-shell`: Move app to `/app` route, add auth-aware navigation (show login/register when unauthenticated, show user menu when authenticated)

## Impact

- **Database**: New `users` table. Migration adds `user_id` to `charts`, `annotations`, `span_annotations`, `annotation_types`, `span_label_types`. Existing data needs migration strategy (assign to a default user or require re-import).
- **Frontend routing**: Current single-page app at `/` moves to `/app`. New public pages: `/`, `/login`, `/register`. New protected page: `/app/settings`.
- **API routes**: All 13 existing API endpoints need auth middleware and user-scoped queries.
- **Dependencies**: Add `next-auth` (v5/beta for App Router), `bcryptjs` for password hashing, `@auth/drizzle-adapter` for database sessions.
- **Environment variables**: New env vars for `NEXTAUTH_SECRET`, `NEXTAUTH_URL`, `GOOGLE_CLIENT_ID`, `GOOGLE_CLIENT_SECRET`.
- **ML Service**: Python FastAPI service needs user context passed via API key or header — training runs and model storage become user-scoped.
- **Docker**: No new services, but env vars need updating in docker-compose.yml.
- **Breaking**: Existing single-user data will need migration to a default user account. **BREAKING** for any existing deployments.
