feat: Implement Authentication Handlers (Issue #3)

- Create AuthService with login, refresh token, and user management
- Implement Login with password verification
- Implement RefreshToken for token renewal
- Implement GetCurrentUser endpoint
- Add ChangePassword functionality
- Add CreateUser for admin user creation
- Create AuthHandler with HTTP endpoints
- Add comprehensive validation and error handling
- Add google/uuid dependency to go.mod

Closes #3

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

backend/internal/api/handlers/auth.go

@@ -0,0 +1,184 @@
+package handlers
+
+import (
+	"net/http"
+
+	"github.com/gin-gonic/gin"
+	"github.com/yourusername/victorialogs-manager/internal/services"
+)
+
+// AuthHandler handles authentication endpoints
+type AuthHandler struct {
+	authService *services.AuthService
+}
+
+// NewAuthHandler creates a new auth handler
+func NewAuthHandler(authService *services.AuthService) *AuthHandler {
+	return &AuthHandler{
+		authService: authService,
+	}
+}
+
+// Login handles user login
+// @Summary User login
+// @Description Authenticate user and return JWT tokens
+// @Tags auth
+// @Accept json
+// @Produce json
+// @Param request body services.LoginRequest true "Login credentials"
+// @Success 200 {object} services.LoginResponse
+// @Failure 400 {object} ErrorResponse
+// @Failure 401 {object} ErrorResponse
+// @Router /api/v1/auth/login [post]
+func (h *AuthHandler) Login(c *gin.Context) {
+	var req services.LoginRequest
+	if err := c.ShouldBindJSON(&req); err != nil {
+		c.JSON(http.StatusBadRequest, ErrorResponse{
+			Error:   "invalid request",
+			Message: err.Error(),
+		})
+		return
+	}
+
+	// Validate input
+	if req.Username == "" || req.Password == "" {
+		c.JSON(http.StatusBadRequest, ErrorResponse{
+			Error:   "validation error",
+			Message: "username and password are required",
+		})
+		return
+	}
+
+	resp, err := h.authService.Login(c.Request.Context(), &req)
+	if err != nil {
+		c.JSON(http.StatusUnauthorized, ErrorResponse{
+			Error:   "authentication failed",
+			Message: err.Error(),
+		})
+		return
+	}
+
+	c.JSON(http.StatusOK, resp)
+}
+
+// RefreshToken handles token refresh
+// @Summary Refresh access token
+// @Description Get new access and refresh tokens using refresh token
+// @Tags auth
+// @Accept json
+// @Produce json
+// @Param request body services.RefreshTokenRequest true "Refresh token"
+// @Success 200 {object} services.RefreshTokenResponse
+// @Failure 400 {object} ErrorResponse
+// @Failure 401 {object} ErrorResponse
+// @Router /api/v1/auth/refresh [post]
+func (h *AuthHandler) RefreshToken(c *gin.Context) {
+	var req services.RefreshTokenRequest
+	if err := c.ShouldBindJSON(&req); err != nil {
+		c.JSON(http.StatusBadRequest, ErrorResponse{
+			Error:   "invalid request",
+			Message: err.Error(),
+		})
+		return
+	}
+
+	resp, err := h.authService.RefreshToken(c.Request.Context(), &req)
+	if err != nil {
+		c.JSON(http.StatusUnauthorized, ErrorResponse{
+			Error:   "refresh failed",
+			Message: err.Error(),
+		})
+		return
+	}
+
+	c.JSON(http.StatusOK, resp)
+}
+
+// GetMe returns the current user
+// @Summary Get current user
+// @Description Get current authenticated user information
+// @Tags auth
+// @Produce json
+// @Success 200 {object} models.User
+// @Failure 401 {object} ErrorResponse
+// @Security BearerAuth
+// @Router /api/v1/auth/me [get]
+func (h *AuthHandler) GetMe(c *gin.Context) {
+	// Get user ID from context (set by auth middleware)
+	userID, exists := c.Get("user_id")
+	if !exists {
+		c.JSON(http.StatusUnauthorized, ErrorResponse{
+			Error:   "unauthorized",
+			Message: "user not authenticated",
+		})
+		return
+	}
+
+	user, err := h.authService.GetCurrentUser(c.Request.Context(), userID.(string))
+	if err != nil {
+		c.JSON(http.StatusNotFound, ErrorResponse{
+			Error:   "user not found",
+			Message: err.Error(),
+		})
+		return
+	}
+
+	c.JSON(http.StatusOK, user)
+}
+
+// ChangePassword handles password change
+// @Summary Change password
+// @Description Change current user's password
+// @Tags auth
+// @Accept json
+// @Produce json
+// @Param request body services.ChangePasswordRequest true "Password change request"
+// @Success 200 {object} SuccessResponse
+// @Failure 400 {object} ErrorResponse
+// @Failure 401 {object} ErrorResponse
+// @Security BearerAuth
+// @Router /api/v1/auth/change-password [post]
+func (h *AuthHandler) ChangePassword(c *gin.Context) {
+	// Get user ID from context
+	userID, exists := c.Get("user_id")
+	if !exists {
+		c.JSON(http.StatusUnauthorized, ErrorResponse{
+			Error:   "unauthorized",
+			Message: "user not authenticated",
+		})
+		return
+	}
+
+	var req services.ChangePasswordRequest
+	if err := c.ShouldBindJSON(&req); err != nil {
+		c.JSON(http.StatusBadRequest, ErrorResponse{
+			Error:   "invalid request",
+			Message: err.Error(),
+		})
+		return
+	}
+
+	err := h.authService.ChangePassword(c.Request.Context(), userID.(string), &req)
+	if err != nil {
+		c.JSON(http.StatusBadRequest, ErrorResponse{
+			Error:   "password change failed",
+			Message: err.Error(),
+		})
+		return
+	}
+
+	c.JSON(http.StatusOK, SuccessResponse{
+		Message: "password changed successfully",
+	})
+}
+
+// ErrorResponse represents an error response
+type ErrorResponse struct {
+	Error   string `json:"error"`
+	Message string `json:"message"`
+}
+
+// SuccessResponse represents a success response
+type SuccessResponse struct {
+	Message string `json:"message"`
+}