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.mixfed.ru: 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/store/admin.go

@@ -0,0 +1,61 @@
+package store
+
+import (
+	"database/sql"
+	"errors"
+	"fmt"
+	"time"
+)
+
+// ErrNoAdmin is returned by GetAdmin when primary setup has not happened yet.
+var ErrNoAdmin = errors.New("no administrator account")
+
+// Admin is the single panel administrator (spec 7.6.1).
+type Admin struct {
+	Username     string
+	PasswordHash string
+	CreatedAt    time.Time
+}
+
+// AdminExists reports whether the administrator account has been created. This
+// doubles as the "primary setup complete" flag: once true, the /setup route is
+// permanently gone (spec 7.6.1).
+func (s *Store) AdminExists() (bool, error) {
+	var n int
+	if err := s.db.QueryRow("SELECT COUNT(*) FROM admin").Scan(&n); err != nil {
+		return false, fmt.Errorf("count admin: %w", err)
+	}
+	return n > 0, nil
+}
+
+// CreateAdmin inserts the administrator row. It fails if one already exists,
+// which — combined with the id=1 constraint — makes admin creation one-shot
+// even under a race between two setup submissions.
+func (s *Store) CreateAdmin(username, passwordHash string) error {
+	_, err := s.db.Exec(
+		"INSERT INTO admin (id, username, password_hash, created_at) VALUES (1, ?, ?, ?)",
+		username, passwordHash, time.Now().UTC().Format(time.RFC3339),
+	)
+	if err != nil {
+		return fmt.Errorf("create admin: %w", err)
+	}
+	return nil
+}
+
+// GetAdmin returns the administrator account, or ErrNoAdmin if setup is pending.
+func (s *Store) GetAdmin() (Admin, error) {
+	var (
+		a         Admin
+		createdAt string
+	)
+	err := s.db.QueryRow("SELECT username, password_hash, created_at FROM admin WHERE id = 1").
+		Scan(&a.Username, &a.PasswordHash, &createdAt)
+	if errors.Is(err, sql.ErrNoRows) {
+		return Admin{}, ErrNoAdmin
+	}
+	if err != nil {
+		return Admin{}, fmt.Errorf("get admin: %w", err)
+	}
+	a.CreatedAt, _ = time.Parse(time.RFC3339, createdAt)
+	return a, nil
+}