feat(internal): add platform detection and auth packages

- internal/platform: cross-platform path detection, 6 tests
- internal/auth: JWT generation/validation + bcrypt, 8 tests
- 89 total tests, build + vet clean

backend/internal/auth/auth.go

@@ -0,0 +1,102 @@
+// Package auth provides JWT token generation and validation for the Cyrene platform.
+package auth
+
+import (
+	"crypto/rand"
+	"encoding/hex"
+	"fmt"
+	"time"
+
+	"github.com/golang-jwt/jwt/v5"
+	"golang.org/x/crypto/bcrypt"
+)
+
+// Claims represents the JWT claims for a Cyrene user.
+type Claims struct {
+	jwt.RegisteredClaims
+	UserID   string `json:"user_id"`
+	Username string `json:"username"`
+	Role     string `json:"role"` // "admin" | "user"
+}
+
+// TokenManager handles JWT creation and validation.
+type TokenManager struct {
+	secret []byte
+	expiry time.Duration
+}
+
+// NewTokenManager creates a new TokenManager.
+// secret must be at least 32 characters.
+func NewTokenManager(secret string, expiryHours int) *TokenManager {
+	return &TokenManager{
+		secret: []byte(secret),
+		expiry: time.Duration(expiryHours) * time.Hour,
+	}
+}
+
+// Generate creates a signed JWT token for the given user.
+func (tm *TokenManager) Generate(userID, username, role string) (string, error) {
+	now := time.Now()
+	claims := &Claims{
+		RegisteredClaims: jwt.RegisteredClaims{
+			Issuer:    "cyrene",
+			Subject:   userID,
+			IssuedAt:  jwt.NewNumericDate(now),
+			ExpiresAt: jwt.NewNumericDate(now.Add(tm.expiry)),
+			ID:        generateTokenID(),
+		},
+		UserID:   userID,
+		Username: username,
+		Role:     role,
+	}
+
+	token := jwt.NewWithClaims(jwt.SigningMethodHS256, claims)
+	return token.SignedString(tm.secret)
+}
+
+// Validate parses and validates a JWT token string.
+// Returns the Claims if valid, or an error.
+func (tm *TokenManager) Validate(tokenStr string) (*Claims, error) {
+	token, err := jwt.ParseWithClaims(tokenStr, &Claims{}, func(token *jwt.Token) (any, error) {
+		if _, ok := token.Method.(*jwt.SigningMethodHMAC); !ok {
+			return nil, fmt.Errorf("unexpected signing method: %v", token.Header["alg"])
+		}
+		return tm.secret, nil
+	})
+	if err != nil {
+		return nil, fmt.Errorf("parse token: %w", err)
+	}
+
+	claims, ok := token.Claims.(*Claims)
+	if !ok || !token.Valid {
+		return nil, fmt.Errorf("invalid token")
+	}
+
+	return claims, nil
+}
+
+// IsAdmin checks if the claims belong to an admin user.
+func (c *Claims) IsAdmin() bool {
+	return c.Role == "admin"
+}
+
+// HashPassword creates a bcrypt hash of the password.
+func HashPassword(password string) (string, error) {
+	bytes, err := bcrypt.GenerateFromPassword([]byte(password), bcrypt.DefaultCost)
+	if err != nil {
+		return "", fmt.Errorf("hash password: %w", err)
+	}
+	return string(bytes), nil
+}
+
+// CheckPassword compares a password with its bcrypt hash.
+func CheckPassword(password, hash string) bool {
+	err := bcrypt.CompareHashAndPassword([]byte(hash), []byte(password))
+	return err == nil
+}
+
+func generateTokenID() string {
+	b := make([]byte, 16)
+	rand.Read(b)
+	return hex.EncodeToString(b)
+}