security: Add proper authentication, RBAC, and tenant isolation

- Add password hashing with bcrypt
- Add AuthService with proper login
- Add password strength validation
- Add RBAC middleware (AdminOnly, ManagerOrAdmin)
- Add tenant isolation middleware
- Update CRM handler with tenant filtering
- Add JWT fallback for development mode
- Add user context helpers
- Build successful

backend/auth/auth.go

@@ -38,6 +38,16 @@ func (c Claims) Valid() error {
 	return nil
 }
 
+// HasRole kontrollerar om användaren har en specifik roll
+func (c Claims) HasRole(role string) bool {
+	for _, r := range c.Roles {
+		if r == role {
+			return true
+		}
+	}
+	return false
+}
+
 // ── Context Key ────────────────────────────────────────────────────────────
 type contextKey int