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/token.go

@@ -0,0 +1,20 @@
+package web
+
+import (
+	"crypto/rand"
+	"encoding/base64"
+)
+
+// randomToken returns a URL-safe token with at least nBytes*8 bits of entropy
+// drawn from crypto/rand. Setup and session tokens both use this; the setup
+// token needs >=128 bits (spec 7.6.1), so callers pass nBytes >= 16.
+//
+// It panics if the system RNG fails: that is unrecoverable and must never be
+// papered over with a weak fallback for a security token.
+func randomToken(nBytes int) string {
+	b := make([]byte, nBytes)
+	if _, err := rand.Read(b); err != nil {
+		panic("crypto/rand failed: " + err.Error())
+	}
+	return base64.RawURLEncoding.EncodeToString(b)
+}