feat(security): harden the API, trust Hiboutik for deletions, lock syncs (#3)

- Sync deletions: a 2xx Hiboutik answer is the truth, even an empty list; a failed call (HTTP error or non-list body) deletes nothing (HiboutikClient.getList).
- Validate the environment at startup (HIBOUTIK_BASE_URL must be HTTPS), global ValidationPipe, Helmet, CORS opt-in, HTTP timeouts, rate limiting (60 req/min/IP, 5 on POST /sync), AllExceptionsFilter.
- Postgres advisory lock so only one synchronization runs at a time across instances; syncAll is private, syncs start through startSync.
- Fix the Jest moduleNameMapper; document the validation and security rules in CLAUDE.md.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

src/config/env.validation.ts

@@ -0,0 +1,51 @@
+import { plainToInstance } from 'class-transformer';
+import { IsInt, IsNotEmpty, IsOptional, IsString, IsUrl, Max, Min, validateSync } from 'class-validator';
+
+/** Environment variables read by the app. A missing or malformed required one stops the startup. */
+export class EnvironmentVariables {
+  @IsString()
+  @IsNotEmpty()
+  DATABASE_URL!: string;
+
+  /** HTTPS only: the Hiboutik credentials are sent with HTTP Basic auth. */
+  @IsUrl({ protocols: ['https'], require_protocol: true })
+  HIBOUTIK_BASE_URL!: string;
+
+  @IsString()
+  @IsNotEmpty()
+  HIBOUTIK_USERNAME!: string;
+
+  @IsString()
+  @IsNotEmpty()
+  HIBOUTIK_PASSWORD!: string;
+
+  @IsOptional()
+  @IsInt()
+  @Min(1)
+  @Max(65535)
+  PORT?: number;
+
+  /** Comma-separated list of origins allowed by CORS. Empty or unset: CORS stays disabled. */
+  @IsOptional()
+  @IsString()
+  CORS_ORIGINS?: string;
+
+  /** Number of reverse proxies in front of the app (so the real client IP is used for rate limiting). Unset: no proxy. */
+  @IsOptional()
+  @IsInt()
+  @Min(1)
+  @Max(10)
+  TRUST_PROXY?: number;
+}
+
+/** `ConfigModule` validation hook: fails fast with the names of the faulty variables (never their values, they may be secrets). */
+export function validateEnv(config: Record<string, unknown>): Record<string, unknown> {
+  const validated = plainToInstance(EnvironmentVariables, config, { enableImplicitConversion: true });
+  const errors = validateSync(validated, { whitelist: false });
+
+  if (errors.length > 0) {
+    const details = errors.map((error) => `${error.property}: ${Object.values(error.constraints ?? {}).join(', ')}`).join('\n  ');
+    throw new Error(`Invalid environment configuration:\n  ${details}`);
+  }
+  return config;
+}