fix(tests): use per-test in-memory DB to prevent shared state conflicts

The testDB() function used cache=shared which caused UNIQUE constraint
failures when multiple tests seeded the same chip_sets row.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/financial/engine_test.go

@@ -15,7 +15,7 @@ import (
 // testDB creates an in-memory SQLite database with the required schema.
 func testDB(t *testing.T) *sql.DB {
 	t.Helper()
-	db, err := sql.Open("libsql", "file::memory:?cache=shared")
+	db, err := sql.Open("libsql", "file:"+t.Name()+"?mode=memory")
 	if err != nil {
 		t.Fatalf("open test db: %v", err)
 	}