fix(tests): address review gaps in test suite

- Add tsconfig.test.json + typecheck:test script so test/ is type-checked;
  fixes real type errors it surfaced (hashedPw typing, PartialUser|ClientUser
  narrowing for .email).
- Add HTTP-level auth-http.test.ts for the api backend covering
  login/session-cookie/logout/clear-sessions/bearer-apikey flows, since
  ApiInterface doesn't implement the session/login methods needed to run the
  shared session/auth suites directly.
- Make the readonly-db test in shared/users.ts actually construct a readonly
  db instance (previously a no-op that never ran) via a new getReadonlyDb
  parameter, wired up for both sqlite and api backends.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

tsconfig.test.json

@@ -0,0 +1,7 @@
+{
+    "extends": "./tsconfig.json",
+    "compilerOptions": {
+        "rootDir": "."
+    },
+    "include": ["src/**/*.ts", "src/**/*.tsx", "test/**/*.ts"]
+}