Update README.md with user accounts feature description (Task 15.2)

Add new 'User Accounts & Authentication' section describing:
- Multi-user support with per-user data isolation
- Auth.js v5 with credentials and Google OAuth sign-in
- Registration with email validation and password requirements
- Settings page for profile management and account deletion
- Default admin account seeding

Remove outdated single-user limitation from Known Limitations section.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

README.md

@@ -29,6 +29,17 @@ Candle Annotator is a complete machine learning platform for candlestick pattern
 
 ## Features
 
+### User Accounts & Authentication
+- **Multi-User Support**: Each user has isolated data with per-user workspace
+- **Auth.js v5 Integration**: Flexible authentication system supporting multiple sign-in methods
+- **Sign-In Methods**:
+  - **Credentials (Email/Password)**: Traditional email and password authentication with bcryptjs hashing
+  - **Google OAuth**: One-click sign-in with Google accounts
+- **Registration**: Self-service account creation with email validation and password requirements (minimum 8 characters)
+- **Settings Page**: Update display name, change password for credential users, or delete account with confirmation
+- **Default Admin Account**: Database seeding with default admin credentials for initial setup
+- **Per-User Data Isolation**: All charts, annotations, and ML models are scoped to individual users
+
 ### Data Management
 - **CSV Upload**: Import OHLC data with support for both Unix timestamps and date strings
   - **Replace Mode**: Uploading a new CSV deletes all old candles and replaces them with new data
@@ -476,7 +487,6 @@ candle_annotator/
 
 ### Known Limitations
 
-- **Single User**: No authentication or concurrent access support
 - **No Undo**: Can only delete annotations, not undo placement
 - **Memory**: Large CSV files (100k+ rows) may cause slow uploads
 - **Line Snapping**: Lines don't snap to candles, free-form placement only