Run SQLite in WAL mode with a busy timeout the driver reads (closes #123)

The default database URL asked for WAL mode with `_journal_mode` and
`_busy_timeout`, which the SQLite driver does not read, so the database
ran in rollback-journal mode. The default now uses the driver's
`_pragma=journal_mode(WAL)` and `_pragma=busy_timeout(5000)` parameters,
in one constant that both the config default and the database fallback
use. A new test opens a file database with those parameters through
db.New and checks the journal mode and busy timeout. README.md's
database text now matches, and an in-memory test database drops the
same ignored parameter.

Model: opus-5-5

internal/db/db.go

@@ -116,6 +116,13 @@ func (database *Database) connect(ctx context.Context) error {
 		return fmt.Errorf("enable foreign keys: %w", err)
 	}
 
+	_, err = database.conn.ExecContext(
+		ctx, "PRAGMA busy_timeout = 5000",
+	)
+	if err != nil {
+		return fmt.Errorf("set busy timeout: %w", err)
+	}
+
 	return database.runMigrations(ctx)
 }