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/web/handlers_auth.go

@@ -0,0 +1,113 @@
+package web
+
+import (
+	"errors"
+	"net/http"
+	"strings"
+
+	"codeberg.org/mix/selfpost/internal/store"
+	"golang.org/x/crypto/bcrypt"
+)
+
+// sessionCookie is the name of the panel session cookie.
+const sessionCookie = "selfpost_session"
+
+// handleLogin serves the login form (GET) and authenticates (POST). Until an
+// administrator exists there is nobody to log in, so it points at setup.
+func (s *Server) handleLogin(w http.ResponseWriter, r *http.Request) {
+	exists, err := s.store.AdminExists()
+	if err != nil {
+		http.Error(w, "internal error", http.StatusInternalServerError)
+		return
+	}
+	if !exists {
+		// No admin yet: login is meaningless. Send a clear message rather than
+		// a failing form.
+		s.render(w, http.StatusOK, "login", map[string]any{
+			"Title":     "SelfPost — Sign in",
+			"SetupHint": true,
+		})
+		return
+	}
+
+	switch r.Method {
+	case http.MethodGet:
+		s.renderLogin(w, http.StatusOK, "")
+	case http.MethodPost:
+		s.submitLogin(w, r)
+	default:
+		w.Header().Set("Allow", "GET, POST")
+		http.Error(w, "method not allowed", http.StatusMethodNotAllowed)
+	}
+}
+
+func (s *Server) renderLogin(w http.ResponseWriter, status int, formErr string) {
+	s.render(w, status, "login", map[string]any{
+		"Title": "SelfPost — Sign in",
+		"Error": formErr,
+	})
+}
+
+func (s *Server) submitLogin(w http.ResponseWriter, r *http.Request) {
+	// Brute-force throttle by client IP (spec 7.6.5).
+	if !s.loginLimiter.Allow(clientIP(r)) {
+		s.renderLogin(w, http.StatusTooManyRequests, "Too many attempts. Please wait and try again.")
+		return
+	}
+	if err := r.ParseForm(); err != nil {
+		s.renderLogin(w, http.StatusBadRequest, "Invalid form submission.")
+		return
+	}
+	username := strings.TrimSpace(r.PostFormValue("username"))
+	password := r.PostFormValue("password")
+
+	admin, err := s.store.GetAdmin()
+	if err != nil {
+		if !errors.Is(err, store.ErrNoAdmin) {
+			logf("panel: login: get admin failed: %v", err)
+		}
+		s.renderLogin(w, http.StatusUnauthorized, "Invalid username or password.")
+		return
+	}
+
+	// Always run bcrypt so timing does not distinguish "wrong user" from
+	// "wrong password", and compare the username too.
+	pwErr := bcrypt.CompareHashAndPassword([]byte(admin.PasswordHash), []byte(password))
+	if username != admin.Username || pwErr != nil {
+		s.renderLogin(w, http.StatusUnauthorized, "Invalid username or password.")
+		return
+	}
+
+	token := s.sessions.Create(admin.Username)
+	http.SetCookie(w, &http.Cookie{
+		Name:     sessionCookie,
+		Value:    token,
+		Path:     "/",
+		HttpOnly: true,
+		Secure:   s.cfg.CookieSecure,
+		SameSite: http.SameSiteLaxMode,
+	})
+	http.Redirect(w, r, "/", http.StatusSeeOther)
+}
+
+// handleLogout destroys the session and clears the cookie.
+func (s *Server) handleLogout(w http.ResponseWriter, r *http.Request) {
+	if r.Method != http.MethodPost {
+		w.Header().Set("Allow", "POST")
+		http.Error(w, "method not allowed", http.StatusMethodNotAllowed)
+		return
+	}
+	if c, err := r.Cookie(sessionCookie); err == nil {
+		s.sessions.Destroy(c.Value)
+	}
+	http.SetCookie(w, &http.Cookie{
+		Name:     sessionCookie,
+		Value:    "",
+		Path:     "/",
+		MaxAge:   -1,
+		HttpOnly: true,
+		Secure:   s.cfg.CookieSecure,
+		SameSite: http.SameSiteLaxMode,
+	})
+	http.Redirect(w, r, "/login", http.StatusSeeOther)
+}