Feat: Rate-limit login endpoint to block brute-force attacks

After LOGIN_MAX_FAILURES consecutive failed attempts from the same source
IP within LOGIN_WINDOW_SECONDS, POST /api/v1/auth/token returns HTTP 429
with a Retry-After header for LOGIN_COOLDOWN_SECONDS. A successful login
resets the counter. Trusted upstream proxy IPs/CIDRs can be declared via
LOGIN_TRUSTED_PROXY_IPS so X-Forwarded-For is honoured correctly behind
nginx ingress or similar reverse proxies.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.specify/feature.json

@@ -1 +1,3 @@
-{"feature_directory":"specs/008-postgres-integration-tests"}
+{
+  "feature_directory": "specs/009-login-rate-limiting"
+}