✨ feat: implement user authentication system with JWT and PostgreSQL

Added comprehensive user management system:
- User registration with validation (3-50 char username, 6+ char password)
- JWT-based authentication with bcrypt password hashing
- Admin authentication with master password
- Password reset workflow with admin flagging
- PostgreSQL repository implementation
- SQLite repository for testing
- Unified authentication service interface

API Endpoints:
- POST /api/v1/auth/register - User registration
- POST /api/v1/auth/login - User/admin authentication
- POST /api/v1/auth/password-reset/request - Request password reset
- POST /api/v1/auth/password-reset/complete - Complete password reset
- POST /api/v1/auth/validate - JWT token validation

Security Features:
- Password hashing with bcrypt
- JWT token generation and validation
- Admin claims in JWT tokens
- Configurable token expiration
- Input validation for all endpoints

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

cmd/server/main.go

@@ -1,7 +1,7 @@
 // Package main provides the dance-lessons-coach server entry point
 //
 //	@title			dance-lessons-coach API
-//	@version		1.2.0
+//	@version		1.4.0
 //	@description	API for dance-lessons-coach service providing greeting functionality
 //	@termsOfService	http://swagger.io/terms/
 
@@ -12,9 +12,14 @@
 //	@license.name	MIT
 //	@license.url	https://opensource.org/licenses/MIT
 
-//	@host		localhost:8080
-//	@BasePath	/api
-//	@schemes	http https
+//	@host						localhost:8080
+//	@BasePath					/api
+//	@schemes					http https
+//
+//	@securityDefinitions.apikey	BearerAuth
+//	@in							header
+//	@name						Authorization
+//	@description				JWT authentication using Bearer token. Format: Bearer <token>
 
 package main