Make Analytics Engine optional for free-tier hosting

[buren]

Summary

Analytics Engine requires the Cloudflare Workers Paid plan ($5/mo). This PR makes it fully optional so the app can be deployed and run on the free tier — including via the "Deploy to Cloudflare" button.

• Guard trackLinkRedirect to silently skip when the REDIRECTS binding is absent
• Remove analytics_engine_datasets from wrangler.jsonc so one-click deploy works on free accounts
• Add REDIRECTS? to the Env type so the code compiles with or without the binding
• Update README: mark analytics as optional, change ANALYTICS_API_TOKEN/ACCOUNT_ID to non-required, add "Enabling Analytics" section with opt-in instructions

The stats API endpoint and Slack /ishere stats command were already guarded — they return "not configured" when the analytics secrets are missing.

Test plan

• New unit test: trackLinkRedirect silently returns when REDIRECTS is undefined (no errors logged)
• All 229 existing tests pass
• Pre-existing TS type errors unchanged (not introduced by this PR)