feat(train): signing in to Jellyfin and marking playlists (ADR 0030, D47)

Music's decisions as D47, and its first part: the music screen from the
account sheet, on Android, signs in to the person's Jellyfin with Quick
Connect, else a password, and lists their playlists of music to mark
for running. The token is in the keyring, the server and the marks in
the person's music.json, nothing on Train's server.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

docs/adr/0030-halkyon-train.md

@@ -986,6 +986,59 @@ Strava. What was found, 27 Oct:
   read Train's database; a role of its own for it is left for later. restic now forgets
   snapshots grouped by host alone, so that the snapshots before Train's dump joined
   them aren't kept for good as a group of their own.
+- **D47 Music from Jellyfin** (recommended, the developer's; 29 Oct; D4's how):
+  - **Signing in**: with Jellyfin's Quick Connect (Train shows a code, the person
+    approves it in another Jellyfin client, Train gets a token of its own), else with
+    the user name and password where the server has Quick Connect off (over the
+    password alone, which always passes through Train, and over an API key, which only
+    an administrator makes and which carries an administrator's rights).
+  - **Which tracks**: those of the Jellyfin playlists the person marks for running in
+    Train's settings (over the whole library, hours of downloads and analysis on a large
+    one, and over genres, only as good as the library's tags).
+  - **Kept on this phone only**: the token in secure storage, the server's address, the
+    marked playlists and each track's tempo in a file of the person's on the device,
+    nothing on Train's server (over sealed, synced rows, a synced model and its
+    migration so that a second phone skips the analysis; music plays on phones only,
+    D5). A second phone signs in and analyses again.
+  - **Offline**: the marked playlists' tracks are downloaded on unmetered networks only
+    and kept up with the playlists; the settings show their size and delete them (over
+    fetching what a run needs as it starts, which makes the start wait on the network
+    and a free run guess its length, and over streaming, which D4 ruled out).
+  - **Tempo by Train's own analysis, in Kotlin** (over aubio, GPL-3 C, which needs a
+    native build per ABI; TarsosDSP, GPL-3 Java, unmaintained since 2021; Essentia,
+    AGPL-3 C++, tens of MB and a heavy build): Android's decoder reads a minute of the
+    downloaded track, an onset envelope and its autocorrelation give the beats per
+    minute, tested on the JVM with beats of known tempo. A BPM tag the file carries is
+    taken first.
+  - **Played by Media3's ExoPlayer in the recording service** (Apache-2.0, AndroidX;
+    over just_audio and audio_service in Flutter, whose engine may stop with the
+    activity and whose player would talk to the service's cues and plan runner across
+    the channel): beside the cues, which duck it, and the plan runner, it plays on with
+    the screen off and picks the next track there.
+  - **Cadence followed**: the one the step detector measures, smoothed over 30 s,
+    without a plan and for a step aiming at heart rate or the like; for a step aiming
+    at a pace, the cadence the person's own past runs show at that pace (steps against
+    speed), the measured one until three runs show it (over the measured cadence alone,
+    which never pulls the person towards the step's pace, and over a cadence set by
+    hand). A track fits at its tempo, its half or its double, sped up or slowed down by
+    at most **6 %** with the pitch kept (over 10 %, where voices start to sound off, and
+    over none, which needs a large playlist). A metronome at the cadence plays where no
+    library is set (D4).
+  - In parts: 10a signing in to Jellyfin and marking playlists in the settings; 10b
+    downloading the marked tracks and working out their tempo; 10c playing them in a
+    recording, by cadence, and the metronome.
+  Built: 10a. *Music* in the account sheet, on Android: the server's address (https
+  where none is typed; a path kept, as for a server under `/jellyfin`), then Quick
+  Connect's code, asked about every 3 s and given up after 9 minutes (Jellyfin forgets
+  a request after ten), or *Use a password*, which a server with Quick Connect off
+  asks for at once; Quick Connect as Jellyfin 10.9 and later have it. Train signs in
+  as the client "Halkyon Train" with a device ID of its own, made once and kept, so
+  that the server lists it as a device of the person's. The token is in the keyring
+  under the person's ID, and the tokens of people the device no longer keeps are
+  forgotten as the library opens; a token Jellyfin takes back signs the person out
+  on the phone too. Signing out ends the token on the server where it answers. The
+  person's playlists of music are listed (a playlist naming no type counts as one,
+  one of videos doesn't) and marked, the marked ones by ID and name in `music.json`.
 
 ## Consequences