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>

README.md

@@ -30,8 +30,10 @@ simultaneous listeners), not for public broadcasting.
   jingles (noon, snack time, ...) played once when their time comes.
 - **Smooth playback**: a 3 s crossfade between tracks.
 - **Built-in web player** at `http://localhost:8000/`: now playing (linked to
-  its source page), track history, skip button, per-track download, volume
-  memory, live auto-reconnect, prefetch progress, and a synthwave look.
+  its source page — a Bandcamp/YouTube page for yt-dlp tracks, or an on-demand
+  Subsonic share for library tracks), track history and the upcoming queue,
+  skip/restart controls, per-track download, volume memory, live auto-reconnect,
+  prefetch progress, and a synthwave look.
 - **OS media controls**: the player exposes current track metadata and
   play/pause/next through the Media Session API, so it wires into system media
   controls (MPRIS on Linux, macOS Control Center, Windows, mobile lock screen)
@@ -68,7 +70,9 @@ Fill in `.env`:
 - **OpenSubsonic server**: `RADIEO_SUBSONIC_URL` / `USER` / `PASSWORD` and the
   playlist to broadcast in `RADIEO_SUBSONIC_PLAYLIST` (name or id). Works with
   any OpenSubsonic-compatible server (Navidrome, Gonic, Airsonic…). Leave empty
-  to disable this source.
+  to disable this source. To make the player's "source" link work for library
+  tracks, enable sharing on the server (Navidrome: `ND_ENABLESHARING=true`); the
+  link then mints a public share on demand, only when clicked.
 - **Mix**: `RADIEO_WEIGHT_SUBSONIC` / `RADIEO_WEIGHT_YTDLP` /
   `RADIEO_WEIGHT_LISTENBRAINZ` set the relative draw weight of each source
   (`0` disables one).
@@ -137,14 +141,17 @@ time, and serves it over an internal HTTP API:
 - A *prefetch queue* keeps a few ready tracks; a SQLite database under `state/`
   holds play history, the MBID cache, and LRU cache-file retention.
 - API: `GET /next` (annotated Liquidsoap URI), `/fallback.m3u` (already-aired
-  tracks), `/status` (prefetch progress), `/healthz`.
+  tracks), `/status` (prefetch progress), `/queue` (upcoming tracks),
+  `/healthz`, and `POST /share?id=<songId>` (mint a Subsonic share on demand).
 
 **`stream`** (Liquidsoap) pulls `/next` via `request.dynamic`, inserts jingles
 (a `switch` that also handles the time-of-day slots), applies the crossfade and
 `mksafe`, and outputs the MP3. On the same harbor port 8000 it also serves the
-web player and its API: `/nowplaying`, `/history`, `/skip`, `/download`, plus
-`/ingest/status`.
+web player and its API: `/nowplaying`, `/history`, `/skip`, `/restart-track`,
+`/download`, and — proxied from `ingest` — `/queue`, `/ingest/status`, and
+`/share` (which forwards to `ingest`, then redirects to the created share).
 
 Only port **8000** is published to the host. The browser never talks to `ingest`
 directly — the Liquidsoap harbor acts as a small reverse proxy for the data the
-player needs (e.g. `/ingest/status`), keeping everything on a single origin.
+player needs (e.g. `/ingest/status`, `/queue`, `/share`), keeping everything on a
+single origin.