ingest: give every track a source link

The player's "source" link only worked for direct yt-dlp URLs. Two other
cases had no linkable page: ListenBrainz picks resolved via ytsearch1: (the
locator is a search query) and Subsonic library tracks (an opaque song id).

Centralise the rule in Track.page_url and cover both: the yt-dlp fetcher now
records the concrete video URL it resolved into source_url, and a Subsonic
track links to the stream's new /share endpoint, which asks ingest to mint a
public share (createShare) on demand and redirects to it — so a share is only
created when a listener actually clicks, never per played track.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

stream/radio.liq

@@ -372,3 +372,33 @@ harbor.http.register(
     serve_attachment(resp, name)
   end
 )
+
+# Partage Subsonic à la demande. Un morceau de la bibliothèque Subsonic n'a pas
+# d'URL publique : son lien « source » pointe ici avec l'id du morceau. On
+# demande alors à l'ingest (qui détient les identifiants Subsonic) de créer un
+# partage public via createShare, puis on redirige l'auditeur vers l'URL
+# renvoyée. Le partage n'est donc créé que si quelqu'un clique réellement sur le
+# lien — jamais à chaque morceau joué. 404 si l'id manque, 502 si l'ingest ne
+# peut pas partager (partage désactivé côté serveur, injoignable…).
+ingest_share_url = "http://ingest:8080/share"
+harbor.http.register(
+  port=8000, method="GET", "/share",
+  fun(req, resp) -> begin
+    song = list.assoc(default="", "song", req.query)
+    if song == "" then
+      resp.status_code(404)
+      resp.data("missing song id")
+    else
+      body = http.post(data="", timeout=10.0, "#{ingest_share_url}?id=#{url.encode(song)}")
+      share = json.parse(default={url=""}, string.trim(body))
+      if body.status_code == 200 and share.url != "" then
+        resp.status_code(302)
+        resp.header("Location", share.url)
+        resp.data("")
+      else
+        resp.status_code(502)
+        resp.data("share unavailable")
+      end
+    end
+  end
+)