feat: rate-limit /auth/login, /register, /me/password (0.35.0)

A hand-rolled token-bucket limiter (5 req/sec, 10-request burst by
default; AUTH_RATE_PER_SEC/AUTH_RATE_BURST env knobs) gates the three
auth-mutation endpoints. One bucket per AppState so tests stay
isolated. Tower-governor wasn't wired in because the reverse proxy
doesn't yet forward client IPs — a global bucket gives equivalent
brute-force protection until that lands.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.env.example

@@ -29,6 +29,13 @@ COOKIE_DOMAIN=
 # get reaped lazily.
 SESSION_TTL_DAYS=30
 
+# ----- Auth brute-force rate limits -----
+# Token-bucket budget shared across /auth/login, /auth/register, and
+# /auth/me/password. Set per_sec=0 to disable (e.g. behind a
+# rate-limiting reverse proxy that already enforces a budget).
+AUTH_RATE_PER_SEC=5
+AUTH_RATE_BURST=10
+
 # ----- CORS -----
 # Comma-separated origins allowed to call the API with credentials.
 # Default is empty: same-origin only. Set when frontend and backend live