Add _txlock=immediate so batch writes wait instead of dropping (closes #25)

Batch flush paths read before they write, so a deferred transaction starts
as a reader and must upgrade to the write lock on its first INSERT/UPDATE.
When the background maintainer holds the write lock for a WAL checkpoint,
that upgrade fails immediately with "database is locked" and the busy
timeout does not apply, so the batch is dropped. Adding _txlock=immediate
to the DSN makes every transaction take the write lock at BEGIN, so it
waits up to busy_timeout instead of failing.

A regression test drives batch writes against a running checkpoint loop and
fails with "database is locked" without the change.

Model: opus-4-8

internal/database/database.go

@@ -88,9 +88,13 @@ func New(cfg *config.Config, logger *logger.Logger) (*Database, error) {
 	}
 
 	// Per-connection SQLite settings go in the DSN so every pooled connection
-	// gets them, not just the one that runs the Initialize pragmas.
+	// gets them, not just the one that runs the Initialize pragmas. _txlock=
+	// immediate makes every transaction take the write lock at BEGIN. Without it
+	// a transaction that reads before writing starts as a reader and, when it
+	// then writes while another connection holds the write lock, fails at once
+	// with "database is locked" without waiting for _busy_timeout.
 	dsn := fmt.Sprintf(
-		"file:%s?_cache_size=%d&_synchronous=OFF&_busy_timeout=%d&_journal_mode=WAL",
+		"file:%s?_cache_size=%d&_synchronous=OFF&_busy_timeout=%d&_journal_mode=WAL&_txlock=immediate",
 		dbPath,
 		sqliteCacheSizeKiB,
 		sqliteBusyTimeoutMs,