feat: protect write endpoints with JWT auth (story #08)

Add bcrypt user file auth, JWT middleware on all write routes, Pinia
authStore with localStorage persistence, login view with redirect
support, and v-if guards on all CRUD controls and admin nav link.

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

backend/internal/handler/auth_handler.go

@@ -0,0 +1,44 @@
+package handler
+
+import (
+	"net/http"
+
+	"github.com/labstack/echo/v4"
+
+	"osdb/internal/auth"
+)
+
+// AuthHandler handles authentication endpoints.
+type AuthHandler struct {
+	users     map[string]string
+	jwtSecret string
+}
+
+// NewAuthHandler creates an AuthHandler with the given user map and JWT secret.
+func NewAuthHandler(users map[string]string, jwtSecret string) *AuthHandler {
+	return &AuthHandler{users: users, jwtSecret: jwtSecret}
+}
+
+type loginRequest struct {
+	Username string `json:"username"`
+	Password string `json:"password"`
+}
+
+// Login handles POST /api/v1/auth/login.
+func (h *AuthHandler) Login(c echo.Context) error {
+	var req loginRequest
+	if err := c.Bind(&req); err != nil || req.Username == "" {
+		return c.JSON(http.StatusUnauthorized, map[string]string{"error": "invalid credentials"})
+	}
+
+	if !auth.VerifyPassword(h.users, req.Username, req.Password) {
+		return c.JSON(http.StatusUnauthorized, map[string]string{"error": "invalid credentials"})
+	}
+
+	token, err := auth.GenerateJWT(req.Username, h.jwtSecret)
+	if err != nil {
+		return c.JSON(http.StatusInternalServerError, map[string]string{"error": "token generation failed"})
+	}
+
+	return c.JSON(http.StatusOK, map[string]string{"token": token})
+}