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.

songs_test.go

@@ -8,7 +8,7 @@ import (
 func (a *app) seedSong(t *testing.T, submitter int64, title string) int64 {
 	t.Helper()
 	var id int64
-	err := a.pool.QueryRow(context.Background(), `
+	err := a.db.QueryRowContext(context.Background(), `
 		insert into songs (title, artist, genre, audio_file, duration_seconds, submitted_by)
 		values ($1, 'Testiartisti', 'Metal', 'x.ogg', 120, $2) returning id`,
 		title, submitter).Scan(&id)
@@ -21,7 +21,7 @@ func (a *app) seedSong(t *testing.T, submitter int64, title string) int64 {
 func (a *app) seedReview(t *testing.T, songID, reviewerID int64, score int) int64 {
 	t.Helper()
 	var id int64
-	err := a.pool.QueryRow(context.Background(), `
+	err := a.db.QueryRowContext(context.Background(), `
 		insert into reviews (song_id, reviewer_id, score, text)
 		values ($1, $2, $3, 'sanat') returning id`, songID, reviewerID, score).Scan(&id)
 	if err != nil {
@@ -133,8 +133,8 @@ func TestQueueContents(t *testing.T) {
 
 	// Oldest first: a second unreviewed song comes after the first.
 	older := a.seedSong(t, bertta, "Vanhempi")
-	if _, err := a.pool.Exec(ctx,
-		`update songs set created_at = now() - interval '2 days' where id = $1`, older); err != nil {
+	if _, err := a.db.ExecContext(ctx,
+		`update songs set created_at = datetime('now', '-2 days') where id = $1`, older); err != nil {
 		t.Fatal(err)
 	}
 	list, err = a.queue(ctx, aino, 0)
@@ -165,7 +165,7 @@ func TestSongUnlocksWhenTheLastReviewGoes(t *testing.T) {
 		t.Fatal("a reviewed song is still editable")
 	}
 
-	if _, err := a.pool.Exec(ctx, `delete from reviews where id = $1`, reviewID); err != nil {
+	if _, err := a.db.ExecContext(ctx, `delete from reviews where id = $1`, reviewID); err != nil {
 		t.Fatal(err)
 	}
 	d, _ = a.song(ctx, aino, songID)
@@ -192,8 +192,8 @@ func TestEditWindow(t *testing.T) {
 	}
 
 	// Just inside the window.
-	if _, err := a.pool.Exec(ctx,
-		`update reviews set updated_at = now() - interval '29 minutes' where id = $1`,
+	if _, err := a.db.ExecContext(ctx,
+		`update reviews set updated_at = datetime('now', '-29 minutes') where id = $1`,
 		reviewID); err != nil {
 		t.Fatal(err)
 	}
@@ -203,8 +203,8 @@ func TestEditWindow(t *testing.T) {
 	}
 
 	// Past it.
-	if _, err := a.pool.Exec(ctx,
-		`update reviews set updated_at = now() - interval '31 minutes' where id = $1`,
+	if _, err := a.db.ExecContext(ctx,
+		`update reviews set updated_at = datetime('now', '-31 minutes') where id = $1`,
 		reviewID); err != nil {
 		t.Fatal(err)
 	}
@@ -215,17 +215,17 @@ func TestEditWindow(t *testing.T) {
 
 	// The database is the authority, not the Go clock: the update and the delete both refuse.
 	var n int64
-	err = a.pool.QueryRow(ctx, `
-		update reviews set score = 1, updated_at = now()
-		 where id = $1 and reviewer_id = $2 and updated_at > now() - $3::interval
-		 returning id`, reviewID, bertta, editWindow.String()).Scan(&n)
+	err = a.db.QueryRowContext(ctx, `
+		update reviews set score = 1, updated_at = datetime('now')
+		 where id = $1 and reviewer_id = $2 and updated_at > datetime('now', $3)
+		 returning id`, reviewID, bertta, editWindowAgo).Scan(&n)
 	if err == nil {
 		t.Fatal("an expired review was edited")
 	}
-	err = a.pool.QueryRow(ctx, `
+	err = a.db.QueryRowContext(ctx, `
 		delete from reviews
-		 where id = $1 and reviewer_id = $2 and updated_at > now() - $3::interval
-		 returning id`, reviewID, bertta, editWindow.String()).Scan(&n)
+		 where id = $1 and reviewer_id = $2 and updated_at > datetime('now', $3)
+		 returning id`, reviewID, bertta, editWindowAgo).Scan(&n)
 	if err == nil {
 		t.Fatal("an expired review was deleted")
 	}