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/middleware.go

@@ -0,0 +1,51 @@
+package web
+
+import (
+	"context"
+	"net/http"
+)
+
+type ctxKey int
+
+const usernameKey ctxKey = 0
+
+// requireAuth wraps a handler so only requests with a valid session cookie
+// reach it; everyone else is redirected to the login page. The authenticated
+// username is stashed in the request context for downstream handlers.
+func (s *Server) requireAuth(next http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		c, err := r.Cookie(sessionCookie)
+		if err != nil {
+			http.Redirect(w, r, "/login", http.StatusSeeOther)
+			return
+		}
+		username, ok := s.sessions.Lookup(c.Value)
+		if !ok {
+			http.Redirect(w, r, "/login", http.StatusSeeOther)
+			return
+		}
+		ctx := context.WithValue(r.Context(), usernameKey, username)
+		next.ServeHTTP(w, r.WithContext(ctx))
+	})
+}
+
+// currentUser returns the authenticated username from the request context.
+func currentUser(r *http.Request) string {
+	if v, ok := r.Context().Value(usernameKey).(string); ok {
+		return v
+	}
+	return ""
+}
+
+// handleDashboard is the authenticated landing page. Phase 2 shows a minimal
+// shell; domains, applications and the send log arrive in later phases.
+func (s *Server) handleDashboard(w http.ResponseWriter, r *http.Request) {
+	if r.URL.Path != "/" {
+		http.NotFound(w, r)
+		return
+	}
+	s.render(w, http.StatusOK, "dashboard", map[string]any{
+		"Title": "SelfPost",
+		"User":  currentUser(r),
+	})
+}