Replace Postgres with SQLite

Ten members and a handful of songs a week never needed a database server,
and the server was the last thing making this a two-container deployment.
modernc.org/sqlite is pure Go, so CGO_ENABLED=0 survives and the dependency
count is unchanged: pgx out, sqlite in.

The port stayed small because the driver matches $1-style placeholders
against argument ordinals exactly as pgx does, so no query needed rewriting
for parameters. What did change:

- timestamptz becomes timestamp holding UTC 'YYYY-MM-DD HH:MM:SS'. The
  declared type is what makes the driver return time.Time, and the
  fixed-width UTC string is what makes ordering and comparison against
  datetime('now') mean what they say.
- interval has no equivalent: sessions.idle_ttl is seconds, and the review
  edit window travels as a SQLite date modifier string.
- No stddev_pop, so the divisive and unified boards spell the population
  formula out, guarded with max(0.0, ...) because cancellation returns a
  tiny negative when every score is identical.
- foreign_keys is off by default, so the cascades only exist because the
  pragma is set on every connection.

Drops the postgres service, its healthcheck, the depends_on gate, the
startup retry loop and POSTGRES_PASSWORD. ./storage is now the whole
backup. Tests get a fresh database file per test and run everywhere
instead of skipping without TEST_DATABASE_URL.

lyrics.go

@@ -241,14 +241,14 @@ func (a *app) autoFetchLyrics(ctx context.Context, subID int64, title, artist st
 	if lyrics == "" {
 		return
 	}
-	tag, err := a.pool.Exec(ctx,
+	res, err := a.db.ExecContext(ctx,
 		`update submissions set lyrics = $2 where id = $1 and lyrics is null`,
 		subID, cleanLyrics(lyrics))
 	if err != nil {
 		slog.Error("save lyrics", "ctx", "submissions", "error", err, "submission", subID)
 		return
 	}
-	if tag.RowsAffected() > 0 {
+	if affected(res) > 0 {
 		slog.Info("lyrics found", "ctx", "submissions", "submission", subID)
 	}
 }