Split internal/web into subpackages before domain-admin growth.

Lay out view, auth, validate, and handlers under internal/web while keeping
the cmd/panel API unchanged; update roadmap and changelog for web-split closure.

Co-authored-by: Cursor <cursoragent@cursor.com>

internal/web/auth/ratelimit.go

@@ -0,0 +1,55 @@
+package auth
+
+import (
+	"sync"
+	"time"
+)
+
+// rateLimiter is a simple fixed-window per-key counter used to throttle the
+// setup and login routes (security.md). Keys are client IPs.
+type rateLimiter struct {
+	max    int
+	window time.Duration
+
+	mu      sync.Mutex
+	buckets map[string]*rlBucket
+}
+
+type rlBucket struct {
+	count      int
+	windowEnds time.Time
+}
+
+func newRateLimiter(max int, window time.Duration) *rateLimiter {
+	return &rateLimiter{
+		max:     max,
+		window:  window,
+		buckets: make(map[string]*rlBucket),
+	}
+}
+
+func (r *rateLimiter) Allow(key string) bool {
+	now := time.Now()
+	r.mu.Lock()
+	defer r.mu.Unlock()
+
+	b := r.buckets[key]
+	if b == nil || now.After(b.windowEnds) {
+		r.buckets[key] = &rlBucket{count: 1, windowEnds: now.Add(r.window)}
+		r.sweep(now)
+		return true
+	}
+	if b.count >= r.max {
+		return false
+	}
+	b.count++
+	return true
+}
+
+func (r *rateLimiter) sweep(now time.Time) {
+	for k, b := range r.buckets {
+		if now.After(b.windowEnds) {
+			delete(r.buckets, k)
+		}
+	}
+}