fix: more aggressive user agent for URL preview

adding "facebookexternalhit" alongside "embedbot" fixes many errors, such as YouTube Music's:
    "Your browser is deprecated. Please upgrade."

add admin command to clear URL stuck and broken data (per URL currently)

    add command to clear all saved URL previews.
    sync resolver docs.

src/admin/media/commands.rs

@@ -388,3 +388,19 @@ pub(super) async fn get_remote_thumbnail(
 	self.write_str(&format!("```\n{result:#?}\nreceived {len} bytes for file content.\n```"))
 		.await
 }
+
+#[admin_command]
+pub(super) async fn delete_url_preview(&self, url: Option<String>, all: bool) -> Result {
+	if all {
+		self.services.media.clear_url_previews().await;
+
+		return self.write_str("Deleted all cached URL previews.").await;
+	}
+
+	let url = url.expect("clap enforces url is required unless --all");
+
+	self.services.media.remove_url_preview(&url).await?;
+
+	self.write_str(&format!("Deleted cached URL preview for: {url}"))
+		.await
+}

src/admin/media/mod.rs

@@ -108,4 +108,16 @@ pub enum MediaCommand {
 		#[arg(long, default_value("800"))]
 		height: u32,
 	},
+
+	/// Deletes a cached URL preview, forcing it to be re-fetched.
+	/// Use --all to purge all cached URL previews.
+	DeleteUrlPreview {
+		/// The URL to clear from the saved preview data
+		#[arg(required_unless_present = "all")]
+		url: Option<String>,
+
+		/// Purge all cached URL previews
+		#[arg(long, conflicts_with = "url")]
+		all: bool,
+	},
 }

src/admin/query/resolver.rs

@@ -20,7 +20,17 @@ pub enum ResolverCommand {
 		name: Option<String>,
 	},
 
-	/// Flush a specific server from the resolver caches or everything
+	/// Flush a given server from the resolver caches or flush them completely
+	///
+	/// * Examples:
+	///   * Flush a specific server:
+	///
+	///     `!admin query resolver flush-cache matrix.example.com`
+	///
+	///   * Flush all resolver caches completely:
+	///
+	///     `!admin query resolver flush-cache --all`
+	#[command(verbatim_doc_comment)]
 	FlushCache {
 		name: Option<OwnedServerName>,