Update admin theme/layout and refresh README details.

Align the project baseline with the latest admin interface styling and layout structure while documenting setup and usage updates in README.

apps/api/tests/modules/media/test_media_router.py

@@ -0,0 +1,21 @@
+def test_media_requires_signature(client):
+    response = client.get("/api/v1/media/files/avatars/missing.png")
+    assert response.status_code == 422
+
+
+def test_media_file_not_found(client):
+    from app.core.media_signing import build_signed_media_url
+
+    signed_url = build_signed_media_url("/api/v1/media/files/avatars/missing.png")
+    response = client.get(signed_url)
+    assert response.status_code == 404
+    assert response.json()["detail"] == "FILE_NOT_FOUND"
+
+
+def test_media_rejects_non_avatar_path(client):
+    from app.core.media_signing import build_signed_media_url
+
+    signed_url = build_signed_media_url("/api/v1/media/files/other/file.png")
+    response = client.get(signed_url)
+    assert response.status_code == 404
+    assert response.json()["detail"] == "FILE_NOT_FOUND"