Add lyrics: paste, fetch, and read them while reviewing

Lyrics are suggested at submission and never imposed. The conversion worker
makes one LRCLIB lookup with whatever metadata exists, and the waiting page
has a Hae sanoitukset button that re-queries with whatever title and artist
are currently typed — which is the case that matters, since our metadata comes
from ID3 tags and YouTube uploaders. Neither path overwrites typed text.

- lyrics text on both submissions and songs, copied across at publish. Nothing
  has launched, so the column goes into 001_init.sql rather than a migration
- The lock does not cover lyrics: it freezes what the song claims to be, and
  nobody reviewed the lyrics. So the submitter can still fix them afterwards,
  or paste them for an old song a year later
- The review strip gained a second pane: lyrics on the left, review on the
  right, so following the words costs no scrolling. No lyrics means no pane,
  not an empty one. Below 1024px the panes stack
- LRC timestamps are stored but stripped for reading — they belong to the
  player, not the reader
- The lyrics box is capped and scrolls inside itself, so a long song cannot
  stretch the strip past the screen

Fixes a real bug found on the way: saveMetadata cleared any field the request
did not carry, so publishing wiped the lyrics the worker had just fetched.
Fields absent from a request now keep their stored value.

The client identifies itself to LRCLIB as "levyraati" and nothing more.

Tests cover cleanLyrics keeping line breaks, and fetchLyrics against a local
server: synced beats plain, instrumentals and wrong-length takes are skipped,
and a miss is empty with no error.

migrations/001_init.sql

@@ -31,6 +31,9 @@ create table songs (
     artist           text        not null,
     genre            text        not null,
     description      text,
+    -- LRC or plain text, told apart by whether the first line starts with '['. Not covered by the
+    -- lock: nobody reviewed the lyrics.
+    lyrics           text,
     audio_file       text        not null,
     duration_seconds integer     not null,
     source_url       text,
@@ -51,6 +54,7 @@ create table submissions (
     artist      text,
     genre       text,
     description text,
+    lyrics      text,
     created_at  timestamptz not null default now(),
     constraint submissions_status check (
         status in ('queued', 'downloading', 'converting', 'ready', 'failed')