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.

main_test.go

@@ -4,10 +4,7 @@ import (
 	"context"
 	"net/http"
 	"net/http/httptest"
-	"os"
 	"testing"
-
-	"github.com/jackc/pgx/v5/pgxpool"
 )
 
 func TestRequireAdmin(t *testing.T) {
@@ -40,33 +37,19 @@ func TestRequireAdmin(t *testing.T) {
 	}
 }
 
-// Set TEST_DATABASE_URL to run this against a throwaway database.
 func TestMigrateIsIdempotent(t *testing.T) {
-	url := os.Getenv("TEST_DATABASE_URL")
-	if url == "" {
-		t.Skip("TEST_DATABASE_URL not set")
-	}
 	ctx := context.Background()
-	pool, err := pgxpool.New(ctx, url)
-	if err != nil {
-		t.Fatal(err)
-	}
-	defer pool.Close()
+	a := testApp(t) // already migrated once
 
-	if _, err := pool.Exec(ctx, `drop schema public cascade; create schema public`); err != nil {
-		t.Fatal(err)
+	if err := migrate(ctx, a.db); err != nil {
+		t.Fatalf("second migrate: %v", err)
 	}
-	for i := range 2 {
-		if err := migrate(ctx, pool); err != nil {
-			t.Fatalf("migrate run %d: %v", i+1, err)
-		}
-	}
-	if err := sweep(ctx, pool); err != nil {
+	if err := sweep(ctx, a.db); err != nil {
 		t.Fatalf("sweep: %v", err)
 	}
 
 	var n int
-	if err := pool.QueryRow(ctx, `select count(*) from schema_migrations`).Scan(&n); err != nil {
+	if err := a.db.QueryRowContext(ctx, `select count(*) from schema_migrations`).Scan(&n); err != nil {
 		t.Fatal(err)
 	}
 	if n != 1 {