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/middleware/jwt_auth.go

@@ -0,0 +1,27 @@
+package middleware
+
+import (
+	"net/http"
+	"strings"
+
+	"github.com/labstack/echo/v4"
+
+	"osdb/internal/auth"
+)
+
+// JWTAuth returns an Echo middleware that requires a valid Bearer JWT.
+func JWTAuth(secret string) echo.MiddlewareFunc {
+	return func(next echo.HandlerFunc) echo.HandlerFunc {
+		return func(c echo.Context) error {
+			header := c.Request().Header.Get("Authorization")
+			if !strings.HasPrefix(header, "Bearer ") {
+				return c.JSON(http.StatusUnauthorized, map[string]string{"error": "missing token"})
+			}
+			tokenStr := strings.TrimPrefix(header, "Bearer ")
+			if _, err := auth.ValidateJWT(tokenStr, secret); err != nil {
+				return c.JSON(http.StatusUnauthorized, map[string]string{"error": "invalid token"})
+			}
+			return next(c)
+		}
+	}
+}