expand a preview card for YouTube links

A message containing a YouTube link now carries a preview card. Discord
and X build their own card from the URL in the text, so the work is on
the bluesky side: the new preview.go asks YouTube's oEmbed endpoint for
the title, channel and thumbnail and posts them as an
app.bsky.embed.external.

Links are found by pulling candidates out of the text and parsing them
with net/url rather than by matching a URL shaped regexp, so a host like
youtube.com.example.invalid is not mistaken for the real thing.

Bluesky allows one embed per post, so a message with both attachments
and a link keeps the attachments. A failed oEmbed lookup only costs the
card, not the post.

Downloading the thumbnail wants the attachment download path, so it is
split into fetch and downloadImage, and both now go through a client
with a timeout: the event loop is single threaded and a hung request
would stall every later message.

The Dockerfile built main.go by name, which stops working now that
package main spans two files; it builds the package instead.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

output/output.go

@@ -6,6 +6,14 @@ type Image struct {
 	Filename    string
 }
 
+// Preview is a link preview card ("プレビュー窓") to attach to a post.
+type Preview struct {
+	URL         string
+	Title       string
+	Description string
+	Thumb       *Image
+}
+
 // Ref identifies a post an output has already published so it can later be
 // replied to or deleted. The fields are output specific; only the ones the
 // publishing output filled in are meaningful to it.
@@ -24,8 +32,9 @@ func (r Ref) IsZero() bool {
 
 // Post is a single message to distribute.
 type Post struct {
-	Text   string
-	Images []Image
+	Text    string
+	Images  []Image
+	Preview *Preview
 	// ReplyTo is the Ref this same output returned for the post being
 	// replied to, or nil for a top level post.
 	ReplyTo *Ref