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

vitest.api.config.mts

@@ -0,0 +1,11 @@
+import { defineConfig } from 'vitest/config';
+
+export default defineConfig({
+    test: {
+        include: ['tests/api/**/*.api.spec.ts'],
+        globalSetup: ['tests/api/global-setup.ts'],
+        // Headroom for starting a temporary PocketBase inside a test.
+        testTimeout: 20_000,
+        hookTimeout: 20_000,
+    },
+});