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>

specs/009-login-rate-limiting/contracts/auth.md

@@ -0,0 +1,85 @@
+# API Contract: Authentication
+
+## POST /api/v1/auth/token
+
+Authenticates the owner and returns a JWT access token.
+
+**This endpoint is modified by feature 009** to enforce brute-force protection.
+All previous behaviour is preserved. One new response code (429) is added.
+
+### Request
+
+```
+POST /api/v1/auth/token
+Content-Type: application/json
+```
+
+```json
+{
+  "username": "string",
+  "password": "string"
+}
+```
+
+### Responses
+
+#### 200 OK — Credentials accepted
+
+```json
+{
+  "access_token": "<jwt>",
+  "token_type": "bearer",
+  "expires_in": 86400
+}
+```
+
+Side effect: resets the failure counter for the caller's IP address.
+
+---
+
+#### 401 Unauthorized — Credentials rejected
+
+```json
+{
+  "detail": "Invalid credentials",
+  "code": "invalid_credentials"
+}
+```
+
+Side effect: increments the failure counter for the caller's IP address. If the
+counter reaches `LOGIN_MAX_FAILURES`, subsequent requests from this IP will receive
+429 until the cooldown expires.
+
+---
+
+#### 429 Too Many Requests — Source blocked after repeated failures
+
+**This response is new in feature 009.**
+
+```
+HTTP/1.1 429 Too Many Requests
+Retry-After: 900
+Content-Type: application/json
+```
+
+```json
+{
+  "detail": "Too many failed login attempts. Please try again later.",
+  "code": "login_rate_limited"
+}
+```
+
+The `Retry-After` header value is the configured cooldown duration in seconds (default: 900).
+It reflects the maximum possible wait, not the exact remaining lockout time.
+
+No credentials are verified when this response is returned — the request is
+rejected before authentication is attempted.
+
+---
+
+### Notes
+
+- The failure counter is per source IP address (TCP peer, not forwarded headers).
+- Threshold values (`LOGIN_MAX_FAILURES`, `LOGIN_WINDOW_SECONDS`, `LOGIN_COOLDOWN_SECONDS`)
+  are not disclosed in any response.
+- Counters are in-memory and reset on process restart.