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.

render.go

@@ -78,8 +78,8 @@ func (a *app) render(w http.ResponseWriter, r *http.Request, status int, name st
 	p.Version = version
 	if p.Member != nil {
 		// The queue is a worklist, so its size belongs in the nav.
-		a.pool.QueryRow(r.Context(), `
-			select count(*)::int from songs s
+		a.db.QueryRowContext(r.Context(), `
+			select count(*) from songs s
 			 where s.submitted_by <> $1
 			   and not exists (select 1 from reviews r
 			                    where r.song_id = s.id and r.reviewer_id = $1)`,