Phase 2: SQLite persistence, admin setup-link, login/sessions

Implements the secure single-admin panel entry (spec 7.6).

- internal/store: modernc.org/sqlite (pure Go, static build), WAL +
  foreign keys, embedded PRAGMA user_version migrations; schema 0001
  covers admin/settings/domains/applications/send_log/rate_limits (spec 9).
- Setup secret-link (spec 7.6.1): 128-bit crypto/rand token, printed to
  log + /data/setup-token (0600), 10-min TTL with regeneration, per-IP
  rate limit, subtle.ConstantTimeCompare, failures don't invalidate,
  one-time admin form, permanent invalidation once admin exists (/setup 404).
- bcrypt admin password; server-side username/password validation.
- Login + in-memory sessions, crypto-random token, cookie
  HttpOnly/Secure/SameSite (Secure toggleable for dev HTTP), login
  rate limit, auth middleware.
- html/template base layout + setup/login/dashboard, vendored htmx 2.0.4.
- build/entrypoint.sh: fix bind-mounted /data ownership as root before
  supervisord drops to the unprivileged panel user (found via container test).

Verified on selfpost.example.com: go vet/build/test/gofmt clean; e2e curl
of setup+login flows; docker build + run with -v ./data:/data creates the
DB and 0600 token owned by panel.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

internal/web/ratelimit.go

@@ -0,0 +1,62 @@
+package web
+
+import (
+	"sync"
+	"time"
+)
+
+// rateLimiter is a simple fixed-window per-key counter used to throttle the
+// setup and login routes (spec 7.6.1, 7.6.5). Keys are client IPs. It is not a
+// precise sliding window — a coarse backstop against brute-force and log noise
+// is all these routes need.
+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),
+	}
+}
+
+// Allow records an attempt for key and reports whether it is within the limit.
+// The current window is reset lazily once it elapses.
+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
+}
+
+// sweep drops expired buckets so the map cannot grow without bound. Called
+// under the lock while a window is being reset, which is often enough given the
+// low request volume of these routes.
+func (r *rateLimiter) sweep(now time.Time) {
+	for k, b := range r.buckets {
+		if now.After(b.windowEnds) {
+			delete(r.buckets, k)
+		}
+	}
+}