test(api): type-check harness and cover automigrate and auth isolation

- add tsconfig.tests.json and run tsc in test:api
- move vitest ProvidedContext types to provided-context.d.ts
- rename vitest.api.config.ts to .mts
- kill signal-test fixture with SIGTERM first so cleanup still runs
- add tests: no files in pb_migrations, independent user/guest clients
- docs: drop superuser env vars from tests in README, note tsc in testing.md

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EvmtStx737hu8gXe15e55W

tests/support/provided-context.d.ts

@@ -0,0 +1,10 @@
+// Типы значений, которые global-setup отдаёт тестам через `project.provide` / `inject`.
+import 'vitest';
+
+declare module 'vitest' {
+    export interface ProvidedContext {
+        pbUrl: string;
+        pbDataDir: string;
+        pbSuperuser: { email: string; password: string };
+    }
+}