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

tsconfig.tests.json

@@ -0,0 +1,12 @@
+/* Проверка типов стенда API-тестов и вспомогательного кода: запускается из `pnpm test:api`. */
+{
+    "extends": "./tsconfig.json",
+    "compilerOptions": {
+        "strict": true,
+        "noEmit": true,
+        "allowImportingTsExtensions": true,
+        "types": ["node"]
+    },
+    "files": [],
+    "include": ["tests/**/*.ts", "vitest.api.config.mts"]
+}