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.

reports.go

@@ -47,7 +47,7 @@ func (a *app) reportPage(w http.ResponseWriter, r *http.Request) {
 
 // Seeing your own past reports is what stops the same bug arriving four times.
 func (a *app) myReports(ctx context.Context, userID int64) ([]*report, error) {
-	rows, err := a.pool.Query(ctx, `
+	rows, err := a.db.QueryContext(ctx, `
 		select id, body, coalesce(page, ''), resolved_at, created_at
 		  from reports where user_id = $1 order by created_at desc`, userID)
 	if err != nil {
@@ -79,7 +79,7 @@ func (a *app) createReport(w http.ResponseWriter, r *http.Request) {
 	}
 
 	// "Only on my phone" is the most common bug report and this answers it without asking.
-	_, err := a.pool.Exec(r.Context(), `
+	_, err := a.db.ExecContext(r.Context(), `
 		insert into reports (user_id, body, page, user_agent) values ($1, $2, nullif($3, ''), $4)`,
 		me.ID, body, from, clean(r.Header.Get("User-Agent"), 300))
 	if err != nil {
@@ -95,7 +95,7 @@ func (a *app) createReport(w http.ResponseWriter, r *http.Request) {
 // --- admin ---
 
 func (a *app) adminReports(w http.ResponseWriter, r *http.Request) {
-	rows, err := a.pool.Query(r.Context(), `
+	rows, err := a.db.QueryContext(r.Context(), `
 		select rep.id, rep.body, coalesce(rep.page, ''), coalesce(rep.user_agent, ''),
 		       u.name, rep.resolved_at, rep.created_at
 		  from reports rep join users u on u.id = rep.user_id
@@ -130,8 +130,9 @@ func (a *app) resolveReport(w http.ResponseWriter, r *http.Request) {
 		http.NotFound(w, r)
 		return
 	}
-	if _, err := a.pool.Exec(r.Context(),
-		`update reports set resolved_at = case when resolved_at is null then now() end where id = $1`,
+	if _, err := a.db.ExecContext(r.Context(),
+		`update reports set resolved_at = case when resolved_at is null then datetime('now') end
+		  where id = $1`,
 		id); err != nil {
 		adminError(w, "reports", err)
 		return
@@ -147,12 +148,12 @@ func (a *app) adminDeleteSong(w http.ResponseWriter, r *http.Request) {
 		http.NotFound(w, r)
 		return
 	}
-	tag, err := a.pool.Exec(r.Context(), `delete from songs where id = $1`, id)
+	res, err := a.db.ExecContext(r.Context(), `delete from songs where id = $1`, id)
 	if err != nil {
 		adminError(w, "songs", err)
 		return
 	}
-	if tag.RowsAffected() > 0 {
+	if affected(res) > 0 {
 		removeFile(a.audioPath(id))
 		slog.Info("song deleted by admin", "ctx", "songs", "song", id)
 		a.flash(w, "Kappale poistettu.")
@@ -170,9 +171,9 @@ type adminSong struct {
 }
 
 func (a *app) adminSongs(ctx context.Context) ([]adminSong, error) {
-	rows, err := a.pool.Query(ctx, `
+	rows, err := a.db.QueryContext(ctx, `
 		select s.id, s.title, s.artist, u.name,
-		       (select count(*) from reviews r where r.song_id = s.id)::int, s.created_at
+		       (select count(*) from reviews r where r.song_id = s.id), s.created_at
 		  from songs s join users u on u.id = s.submitted_by
 		 order by s.created_at desc`)
 	if err != nil {