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.

migrate.go

@@ -2,12 +2,11 @@ package main
 
 import (
 	"context"
+	"database/sql"
 	"embed"
 	"fmt"
 	"log/slog"
 	"sort"
-
-	"github.com/jackc/pgx/v5/pgxpool"
 )
 
 //go:embed migrations/*.sql
@@ -15,17 +14,17 @@ var migrationFS embed.FS
 
 // migrate applies every migrations/*.sql not yet recorded, in filename order, each in its own
 // transaction. Applied names are the record — a file that changes after it ran is not re-applied.
-func migrate(ctx context.Context, pool *pgxpool.Pool) error {
-	_, err := pool.Exec(ctx, `create table if not exists schema_migrations (
+func migrate(ctx context.Context, db *sql.DB) error {
+	_, err := db.ExecContext(ctx, `create table if not exists schema_migrations (
 		name       text primary key,
-		applied_at timestamptz not null default now()
+		applied_at timestamp not null default (datetime('now'))
 	)`)
 	if err != nil {
 		return fmt.Errorf("create schema_migrations: %w", err)
 	}
 
 	applied := map[string]bool{}
-	rows, err := pool.Query(ctx, `select name from schema_migrations`)
+	rows, err := db.QueryContext(ctx, `select name from schema_migrations`)
 	if err != nil {
 		return fmt.Errorf("read schema_migrations: %w", err)
 	}
@@ -60,19 +59,19 @@ func migrate(ctx context.Context, pool *pgxpool.Pool) error {
 		if err != nil {
 			return err
 		}
-		tx, err := pool.Begin(ctx)
+		tx, err := db.BeginTx(ctx, nil)
 		if err != nil {
 			return err
 		}
-		if _, err := tx.Exec(ctx, string(sql)); err != nil {
-			tx.Rollback(ctx)
+		if _, err := tx.ExecContext(ctx, string(sql)); err != nil {
+			tx.Rollback()
 			return fmt.Errorf("migration %s: %w", name, err)
 		}
-		if _, err := tx.Exec(ctx, `insert into schema_migrations (name) values ($1)`, name); err != nil {
-			tx.Rollback(ctx)
+		if _, err := tx.ExecContext(ctx, `insert into schema_migrations (name) values ($1)`, name); err != nil {
+			tx.Rollback()
 			return err
 		}
-		if err := tx.Commit(ctx); err != nil {
+		if err := tx.Commit(); err != nil {
 			return fmt.Errorf("migration %s: %w", name, err)
 		}
 		slog.Info("migration applied", "ctx", "startup", "name", name)
@@ -82,29 +81,31 @@ func migrate(ctx context.Context, pool *pgxpool.Pool) error {
 
 // sweep runs the startup cleanup from docs/spec.md §4.6. An in-process conversion goroutine dies
 // with the process, so without this those rows say "converting" forever.
-func sweep(ctx context.Context, pool *pgxpool.Pool) error {
-	tag, err := pool.Exec(ctx, `update submissions
+func sweep(ctx context.Context, db *sql.DB) error {
+	res, err := db.ExecContext(ctx, `update submissions
 		set status = 'failed', status_msg = 'interrupted by restart'
 		where status in ('queued', 'downloading', 'converting')`)
 	if err != nil {
 		return err
 	}
-	if n := tag.RowsAffected(); n > 0 {
+	if n := affected(res); n > 0 {
 		slog.Warn("submissions interrupted by restart", "ctx", "startup", "count", n)
 	}
 
 	// ponytail: temp files of swept submissions are unlinked with the row in step 3, once the
 	// pipeline exists and there is something to unlink.
-	if _, err := pool.Exec(ctx,
-		`delete from submissions where created_at < now() - interval '7 days'`); err != nil {
+	if _, err := db.ExecContext(ctx,
+		`delete from submissions where created_at < datetime('now', '-7 days')`); err != nil {
 		return err
 	}
-	if _, err := pool.Exec(ctx, `delete from sessions where expires_at < now()`); err != nil {
+	if _, err := db.ExecContext(ctx,
+		`delete from sessions where expires_at < datetime('now')`); err != nil {
 		return err
 	}
 
 	var failed int
-	err = pool.QueryRow(ctx, `select count(*) from submissions where status = 'failed'`).Scan(&failed)
+	err = db.QueryRowContext(ctx,
+		`select count(*) from submissions where status = 'failed'`).Scan(&failed)
 	if err != nil {
 		return err
 	}