feat(config): add feature flags for optional security features

- Add enablePasswordReset, enableRefreshTokenRotation, enableAuditLogging flags
- All flags default to false for backward compatibility
- Add getOptionalBoolean helper for parsing boolean env vars
- Update .env.example with feature flag documentation

backend/src/config.ts

@@ -0,0 +1,87 @@
+/**
+ * Configuration validation and environment variable management
+ */
+import dotenv from "dotenv";
+
+dotenv.config();
+
+interface Config {
+  port: number;
+  nodeEnv: string;
+  databaseUrl: string;
+  frontendUrl: string;
+  jwtSecret: string;
+  jwtAccessExpiresIn: string;
+  jwtRefreshExpiresIn: string;
+  rateLimitMaxRequests: number;
+  csrfMaxRequests: number;
+  csrfSecret: string | null;
+  // Feature flags - all default to false for backward compatibility
+  enablePasswordReset: boolean;
+  enableRefreshTokenRotation: boolean;
+  enableAuditLogging: boolean;
+}
+
+const getRequiredEnv = (key: string): string => {
+  const value = process.env[key];
+  if (!value || value.trim().length === 0) {
+    throw new Error(`Missing required environment variable: ${key}`);
+  }
+  return value;
+};
+
+const getOptionalEnv = (key: string, defaultValue: string): string => {
+  return process.env[key] || defaultValue;
+};
+
+const getOptionalBoolean = (key: string, defaultValue: boolean): boolean => {
+  const value = process.env[key];
+  if (!value) return defaultValue;
+  return value.toLowerCase() === "true" || value === "1";
+};
+
+const getRequiredEnvNumber = (key: string, defaultValue: number): number => {
+  const value = process.env[key];
+  if (!value) return defaultValue;
+  const parsed = Number(value);
+  if (!Number.isFinite(parsed) || parsed <= 0) {
+    throw new Error(`Invalid value for environment variable ${key}: must be a positive number`);
+  }
+  return parsed;
+};
+
+export const config: Config = {
+  port: getRequiredEnvNumber("PORT", 8000),
+  nodeEnv: getOptionalEnv("NODE_ENV", "development"),
+  databaseUrl: getRequiredEnv("DATABASE_URL"),
+  frontendUrl: getOptionalEnv("FRONTEND_URL", "http://localhost:6767"),
+  jwtSecret: getRequiredEnv("JWT_SECRET"),
+  jwtAccessExpiresIn: getOptionalEnv("JWT_ACCESS_EXPIRES_IN", "15m"),
+  jwtRefreshExpiresIn: getOptionalEnv("JWT_REFRESH_EXPIRES_IN", "7d"),
+  rateLimitMaxRequests: getRequiredEnvNumber("RATE_LIMIT_MAX_REQUESTS", 1000),
+  csrfMaxRequests: getRequiredEnvNumber("CSRF_MAX_REQUESTS", 60),
+  csrfSecret: process.env.CSRF_SECRET || null,
+  // Feature flags - disabled by default for backward compatibility
+  enablePasswordReset: getOptionalBoolean("ENABLE_PASSWORD_RESET", false),
+  enableRefreshTokenRotation: getOptionalBoolean("ENABLE_REFRESH_TOKEN_ROTATION", false),
+  enableAuditLogging: getOptionalBoolean("ENABLE_AUDIT_LOGGING", false),
+};
+
+// Validate JWT_SECRET strength in production
+if (config.nodeEnv === "production") {
+  if (config.jwtSecret.length < 32) {
+    throw new Error("JWT_SECRET must be at least 32 characters long in production");
+  }
+  if (config.jwtSecret === "your-secret-key-change-in-production") {
+    throw new Error("JWT_SECRET must be changed from default value in production");
+  }
+}
+
+// Validate frontend URL format
+try {
+  new URL(config.frontendUrl);
+} catch {
+  throw new Error(`Invalid FRONTEND_URL format: ${config.frontendUrl}`);
+}
+
+console.log("Configuration validated successfully");