fix: a blurred Patreon preview embedded in a viewable post is not downloaded

A post the account can view can show media from a post it can't, such as a
public teaser carrying a paid post's image. Patreon serves that media blurred,
and the post-level gate (current_user_can_view) lets it through. The CDN URL
says so in its render options ({"b": 70, ...}), so extract_media now drops any
item whose URL asks for a blur.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/test_patreon_client.py

@@ -19,6 +19,7 @@ from backend.app.services.patreon_client import (
     PatreonAuthError,
     PatreonClient,
     PatreonDriftError,
+    is_blurred_preview,
     parse_cursor_from_url,
 )
 
@@ -459,3 +460,26 @@ def test_post_is_gated_viewable_and_missing_are_not_gated():
         {"attributes": {"current_user_can_view": None}}
     ) is False
     assert PatreonClient.post_is_gated({}) is False
+
+
+# A locked preview's render options carry a blur ("b"); an ordinary one doesn't.
+BLURRED = ("https://c10.patreonusercontent.com/4/patreon-media/p/post/174446666/"
+           "743caff4c51648c185e8347271e1a7b7/eyJiIjo3MCwidyI6MTA4MH0%3D/1.png?token-hash=x")
+SHARP = ("https://c10.patreonusercontent.com/4/patreon-media/p/post/174446666/"
+         "843caff4c51648c185e8347271e1a7b7/eyJ3IjoxMDgwfQ%3D%3D/1.png?token-hash=x")
+
+
+def test_a_blurred_locked_preview_is_recognised_by_its_render_options():
+    assert is_blurred_preview(BLURRED)
+    assert not is_blurred_preview(SHARP)
+    assert not is_blurred_preview("https://example.com/eyJnot-json/1.png")
+    assert not is_blurred_preview("https://example.com/plain/1.png")
+
+
+def test_a_viewable_post_embedding_a_blurred_preview_skips_just_that_image(client):
+    post = {"id": "174448839", "type": "post", "attributes": {
+        "current_user_can_view": True,
+        "content": f'<p><img src="{BLURRED}"></p><p><img src="{SHARP}"></p>',
+    }, "relationships": {}}
+    items = client.extract_media(post, {})
+    assert [m.url for m in items] == [SHARP]