Picsou login timing attack reveals valid usernames

Picsou is a self-hosted personal finance dashboard. In the affected login implementation, the `/api/auth/login` endpoint reveals whether a username exists through a timing discrepancy. In `backend/src/main/java/com/picsou/controller/AuthController.java`, `AuthController.login()` throws `BadCredentialsException` immediately for an unknown username, before `passwordEncoder.matches(...)`, whereas an existing username with an incorrect password incurs the `bcrypt` comparison cost. Both cases return the same generic `HTTP 401` response, but repeated latency measurements disclose valid usernames without credentials. Login rate limiting, a small user population, and deployments inaccessible from outside the local network limit practical exposure; the weakness does not itself read or modify account data, though enumerated usernames can assist credential stuffing or brute-force attacks. This issue is fixed in repository release 1.0.8.