The login handler had no admission control before
`spawn_blocking(verify_password)`. On Pi-class hardware Argon2id is ~50-
100 ms per attempt, so a few dozen concurrent anonymous POSTs to
/auth/login park every blocking worker, wedging the entire admin API
and any other path that uses `spawn_blocking`.
Adds two cheap, in-process guards modeled on EmailRateLimiter:
* `LoginRateLimiter` — two sliding-window token buckets, one per
`(remote_ip, username)` (burst 5/60s) and one per `username`
(burst 10/15min). Per-(ip, user) defeats a single attacker pounding
one account; per-user defeats credential-stuffing distributed across
many IPs. Username is case-folded so case variants share a bucket.
Maps GC lazily when they cross a soft size cap.
* Per-process `tokio::sync::Semaphore` — caps concurrent Argon2 verifies
during login. Default 2 permits (Pi-class), overridable via
`PICLOUD_LOGIN_ARGON2_PARALLELISM`. Acquired AFTER the bucket check so
attackers can't queue.
Limit check fires before the DB credentials lookup, so even an unknown
username doesn't cost a query. Denied attempts return 429 + Retry-After.
Real client IP comes from the first X-Forwarded-For entry (Caddy is the
trusted single hop); falls back to "unknown" so the per-user bucket
still gates.
4 unit tests cover bucket burst exhaustion, per-user crossing IPs, case
folding, and per-user independence.
Audit ref: security_audit/08_dos_resource.md (H-2 / H-B1).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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