Document all supported environment variables in the example app README

[ikemHood]

What does this PR do?

Updates the example app README to document every environment variable read by example/express-app.ts, as a table with the current default for each. Defaults match the example app code.

How to test?

bun run test — new example README documents all env vars suite asserts each env var read by the example app is listed.

Checklist

• My code follows the code style of this project.
• I have added tests for my changes.
• I have updated the documentation accordingly.
• I have run bun run test and bun run lint locally.

Issue Reference

Closes #273

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[gemini-code-assist]

Code Review

This pull request updates the example/README.md to document all environment variables in a structured table format and introduces a new test suite (tests/example-readme-env-vars.test.ts) to verify that all environment variables are documented. Feedback includes correcting a minor typo in the README and refactoring the test assertions to use regular expressions instead of exact string matches, preventing fragile test failures due to markdown auto-formatting.

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[gemini-code-assist]

There is a minor typo in the description: "Regenerated on every start unset." It should be "Regenerated on every start if unset." for better readability.

Suggested change

	| `SEP10_SIGNING_KEY` | fresh random keypair | SEP-10 challenge signing secret. Regenerated on every start unset. |
	| `SEP10_SIGNING_KEY` | fresh random keypair | SEP-10 challenge signing secret. Regenerated on every start if unset. |

[gemini-code-assist]

These assertions are highly fragile because they expect exact spacing in the Markdown table. If an auto-formatter (like Prettier) runs on README.md and aligns the table columns with extra spaces, these assertions will fail. Using regular expressions with \s* makes the test robust against formatting changes.

Suggested change

	expect(readme).toContain('| `PORT` | `3000` |');
	expect(readme).toContain('| `CHALLENGE_EXPIRATION_SECONDS` | `300` |');
	expect(readme).toContain('| `WATCHERS_ENABLED` | enabled |');
	expect(readme).toMatch(/\|\s*`PORT`\s*\|\s*`3000`\s*\|/);
	expect(readme).toMatch(/\|\s*`CHALLENGE_EXPIRATION_SECONDS`\s*\|\s*`300`\s*\|/);
	expect(readme).toMatch(/\|\s*`WATCHERS_ENABLED`\s*\|\s*enabled\s*\|/);