Add rate limiting to login endpoint to prevent brute force

Apply per-IP rate limiting (5 attempts/minute) to POST /login using
golang.org/x/time/rate. Returns 429 Too Many Requests when exceeded.

Closes #12

internal/middleware/ratelimit_test.go

@@ -0,0 +1,107 @@
+package middleware
+
+import (
+	"log/slog"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+
+	"git.eeqj.de/sneak/upaas/internal/config"
+)
+
+func newTestMiddleware(t *testing.T) *Middleware {
+	t.Helper()
+
+	return &Middleware{
+		log: slog.Default(),
+		params: &Params{
+			Config: &config.Config{},
+		},
+	}
+}
+
+func TestLoginRateLimitAllowsUpToBurst(t *testing.T) {
+	// Reset the global limiter to get clean state
+	loginLimiter = newIPLimiter()
+
+	mw := newTestMiddleware(t)
+
+	handler := mw.LoginRateLimit()(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		w.WriteHeader(http.StatusOK)
+	}))
+
+	// First 5 requests should succeed (burst)
+	for i := range 5 {
+		req := httptest.NewRequest(http.MethodPost, "/login", nil)
+		req.RemoteAddr = "192.168.1.1:12345"
+		rec := httptest.NewRecorder()
+		handler.ServeHTTP(rec, req)
+		assert.Equal(t, http.StatusOK, rec.Code, "request %d should succeed", i+1)
+	}
+
+	// 6th request should be rate limited
+	req := httptest.NewRequest(http.MethodPost, "/login", nil)
+	req.RemoteAddr = "192.168.1.1:12345"
+	rec := httptest.NewRecorder()
+	handler.ServeHTTP(rec, req)
+	assert.Equal(t, http.StatusTooManyRequests, rec.Code, "6th request should be rate limited")
+}
+
+func TestLoginRateLimitIsolatesIPs(t *testing.T) {
+	loginLimiter = newIPLimiter()
+
+	mw := newTestMiddleware(t)
+
+	handler := mw.LoginRateLimit()(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		w.WriteHeader(http.StatusOK)
+	}))
+
+	// Exhaust IP1's budget
+	for range 5 {
+		req := httptest.NewRequest(http.MethodPost, "/login", nil)
+		req.RemoteAddr = "10.0.0.1:1234"
+		rec := httptest.NewRecorder()
+		handler.ServeHTTP(rec, req)
+	}
+
+	// IP1 should be blocked
+	req := httptest.NewRequest(http.MethodPost, "/login", nil)
+	req.RemoteAddr = "10.0.0.1:1234"
+	rec := httptest.NewRecorder()
+	handler.ServeHTTP(rec, req)
+	assert.Equal(t, http.StatusTooManyRequests, rec.Code)
+
+	// IP2 should still work
+	req2 := httptest.NewRequest(http.MethodPost, "/login", nil)
+	req2.RemoteAddr = "10.0.0.2:1234"
+	rec2 := httptest.NewRecorder()
+	handler.ServeHTTP(rec2, req2)
+	assert.Equal(t, http.StatusOK, rec2.Code, "different IP should not be rate limited")
+}
+
+func TestLoginRateLimitReturns429Body(t *testing.T) {
+	loginLimiter = newIPLimiter()
+
+	mw := newTestMiddleware(t)
+
+	handler := mw.LoginRateLimit()(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		w.WriteHeader(http.StatusOK)
+	}))
+
+	// Exhaust burst
+	for range 5 {
+		req := httptest.NewRequest(http.MethodPost, "/login", nil)
+		req.RemoteAddr = "172.16.0.1:5555"
+		rec := httptest.NewRecorder()
+		handler.ServeHTTP(rec, req)
+	}
+
+	req := httptest.NewRequest(http.MethodPost, "/login", nil)
+	req.RemoteAddr = "172.16.0.1:5555"
+	rec := httptest.NewRecorder()
+	handler.ServeHTTP(rec, req)
+	assert.Equal(t, http.StatusTooManyRequests, rec.Code)
+	assert.Contains(t, rec.Body.String(), "Too Many Requests")
+}