legal: close AGPL packaging gaps

Name the copyright holder, ship NOTICE with the image, serve the licence
from the panel footer on every page, and record the vendored htmx licence.

Co-authored-by: Cursor <cursoragent@cursor.com>

internal/web/handlers_license_test.go

@@ -0,0 +1,42 @@
+package web
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+
+	"github.com/mixeme/selfpost/internal/legal"
+)
+
+func TestLicenseHandlerServesAGPL(t *testing.T) {
+	s := &Server{}
+	rec := httptest.NewRecorder()
+	req := httptest.NewRequest(http.MethodGet, "/license", nil)
+	s.handleLicense(rec, req)
+
+	if rec.Code != http.StatusOK {
+		t.Fatalf("status = %d, want 200", rec.Code)
+	}
+	ct := rec.Header().Get("Content-Type")
+	if !strings.HasPrefix(ct, "text/plain") {
+		t.Errorf("Content-Type = %q, want text/plain", ct)
+	}
+	body := rec.Body.String()
+	if !strings.Contains(body, "GNU AFFERO GENERAL PUBLIC LICENSE") {
+		t.Error("response is missing the AGPL title")
+	}
+	if body != string(legal.License) {
+		t.Error("response body does not match legal.License")
+	}
+}
+
+func TestLicenseHandlerRejectsNonGET(t *testing.T) {
+	s := &Server{}
+	rec := httptest.NewRecorder()
+	req := httptest.NewRequest(http.MethodPost, "/license", nil)
+	s.handleLicense(rec, req)
+	if rec.Code != http.StatusMethodNotAllowed {
+		t.Fatalf("status = %d, want 405", rec.Code)
+	}
+}