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/auth/auth.go

@@ -0,0 +1,83 @@
+package auth
+
+import (
+	"bufio"
+	"errors"
+	"os"
+	"strings"
+	"time"
+
+	"github.com/golang-jwt/jwt/v5"
+	"golang.org/x/crypto/bcrypt"
+)
+
+// Claims is the JWT payload.
+type Claims struct {
+	Username string `json:"username"`
+	jwt.RegisteredClaims
+}
+
+// LoadUsers reads username:bcrypt_hash pairs from path.
+// Lines starting with '#' and blank lines are ignored.
+func LoadUsers(path string) (map[string]string, error) {
+	f, err := os.Open(path)
+	if err != nil {
+		return nil, err
+	}
+	defer f.Close()
+
+	users := make(map[string]string)
+	scanner := bufio.NewScanner(f)
+	for scanner.Scan() {
+		line := strings.TrimSpace(scanner.Text())
+		if line == "" || strings.HasPrefix(line, "#") {
+			continue
+		}
+		parts := strings.SplitN(line, ":", 2)
+		if len(parts) != 2 {
+			continue
+		}
+		users[parts[0]] = parts[1]
+	}
+	return users, scanner.Err()
+}
+
+// VerifyPassword checks username+password against the bcrypt hash stored for that user.
+// The peppered input is "username:password".
+func VerifyPassword(users map[string]string, username, password string) bool {
+	hash, ok := users[username]
+	if !ok {
+		return false
+	}
+	return bcrypt.CompareHashAndPassword([]byte(hash), []byte(username+":"+password)) == nil
+}
+
+// GenerateJWT creates a signed HS256 token for username, valid for 1 hour.
+func GenerateJWT(username, secret string) (string, error) {
+	claims := Claims{
+		Username: username,
+		RegisteredClaims: jwt.RegisteredClaims{
+			ExpiresAt: jwt.NewNumericDate(time.Now().Add(time.Hour)),
+			IssuedAt:  jwt.NewNumericDate(time.Now()),
+		},
+	}
+	return jwt.NewWithClaims(jwt.SigningMethodHS256, claims).SignedString([]byte(secret))
+}
+
+// ValidateJWT parses and verifies token, returning its claims.
+func ValidateJWT(tokenStr, secret string) (*Claims, error) {
+	token, err := jwt.ParseWithClaims(tokenStr, &Claims{}, func(t *jwt.Token) (any, error) {
+		if _, ok := t.Method.(*jwt.SigningMethodHMAC); !ok {
+			return nil, errors.New("unexpected signing method")
+		}
+		return []byte(secret), nil
+	})
+	if err != nil {
+		return nil, err
+	}
+	claims, ok := token.Claims.(*Claims)
+	if !ok || !token.Valid {
+		return nil, errors.New("invalid token")
+	}
+	return claims, nil
+}