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/legal/legal.go

@@ -0,0 +1,22 @@
+// Package legal embeds SelfPost's AGPL-3.0 licence text so the panel can
+// serve it at /license (Appropriate Legal Notices) without depending on
+// GitHub or a file on disk at runtime.
+//
+// The copy in this directory must stay identical to the repository-root
+// LICENSE; license_test.go enforces that.
+package legal
+
+import (
+	_ "embed"
+)
+
+//go:embed LICENSE
+var License []byte
+
+// SourceURL is where Corresponding Source for the published upstream
+// releases lives. Operators who ship a modified version must point their
+// users at their own sources instead (NOTICE; AGPL-3.0 §13).
+const SourceURL = "https://github.com/mixeme/selfpost"
+
+// CopyrightLine is the short copyright notice shown in the panel footer.
+const CopyrightLine = "Copyright © 2026 Mikhail Yenuchenko"