fix(db): transactions wait for the write lock instead of failing (FLAW-23)

A load test with 16 concurrent writers lost 12 of 480 writes with
SQLITE_BUSY: transactions that read before writing cannot be helped by
busy_timeout once another connection writes. _txlock=immediate makes
every transaction take the write lock at BEGIN, so it queues instead.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/storage/sqlite.go

@@ -32,9 +32,12 @@ func NewStore(dbPath string) (*Store, error) {
 		}
 	}
 
+	// _txlock=immediate: transactions take the write lock when they begin. A transaction that reads
+	// first and writes later would otherwise fail at once with SQLITE_BUSY while another one writes
+	// (busy_timeout cannot help there); now it simply waits its turn (FLAW-23).
 	dsn := dbPath
 	if !strings.Contains(dsn, "?") {
-		dsn += "?_pragma=busy_timeout(10000)&_pragma=journal_mode(WAL)&_pragma=synchronous(NORMAL)&_pragma=foreign_keys(1)"
+		dsn += "?_pragma=busy_timeout(10000)&_pragma=journal_mode(WAL)&_pragma=synchronous(NORMAL)&_pragma=foreign_keys(1)&_txlock=immediate"
 	}
 
 	db, err := sql.Open("sqlite", dsn)