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>

test/app.e2e-spec.ts

@@ -27,12 +27,12 @@ describe('AppController (e2e)', () => {
   });
 
   it('POST /sync starts a synchronization (202)', () => {
-    hiboutikService.startSync.mockReturnValue(true);
+    hiboutikService.startSync.mockResolvedValue(true);
     return request(app.getHttpServer()).post('/sync').expect(202).expect({ message: 'Hiboutik synchronization started' });
   });
 
   it('POST /sync answers 409 when a synchronization is already running', () => {
-    hiboutikService.startSync.mockReturnValue(false);
+    hiboutikService.startSync.mockResolvedValue(false);
     return request(app.getHttpServer()).post('/sync').expect(409);
   });