refactor(site): drop RenderData's url field, source_url already carries it

Every adapter built RenderData { url } from the exact value it then moved into Fetched.source_url — a copy of the same canonical URL held in two places, feeding one placeholder. caption_with reads {url} from self.source_url now and the field is gone from the struct (bsky's and twitter's halves of this change landed with their caption commits; their render builders no longer set it either).

crates/x-media/src/site/pixiv/interface.rs

@@ -289,7 +289,6 @@ impl From<Illustration> for Fetched {
             .collect::<Vec<_>>()
             .join(" ");
         let render_data = Some(crate::site::RenderData {
-            url: url.clone(),
             author: encode_text(&illustration.author).into_owned(),
             author_url: author_url.clone(),
             title: encode_text(&illustration.title).into_owned(),