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

package.json

@@ -7,7 +7,7 @@
         "build": "ng build",
         "watch": "ng build --watch --configuration development",
         "test": "ng test",
-        "test:api": "vitest run --config vitest.api.config.ts",
+        "test:api": "tsc -p tsconfig.tests.json && vitest run --config vitest.api.config.mts",
         "lint": "eslint .",
         "lint:fix": "eslint . --fix",
         "serve:ssr:pchelpcrm": "node dist/pchelpcrm/server/server.mjs",