Harden production checklist: dedupe live-data routes and align serializers.

Pin Mathieu's data-path checklist in docs and PR template, remove dead main.py fast/slow handlers, unify orjson via _live_data_json_bytes, and bound LiveUAMap Playwright defaults.

Co-authored-by: Cursor <cursoragent@cursor.com>

.github/pull_request_template.md

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+## Summary
+
+<!-- What changed and why (1–3 bullets). -->
+
+## Test plan
+
+- [ ] <!-- How you verified the change -->
+
+## Production hardening (data path / fetchers / unattended deploys only)
+
+If this PR touches the data path, fetchers, or live-data APIs, walk through [docs/production-hardening.md](../docs/production-hardening.md) and note any N/A items here.
+
+- [ ] Checklist reviewed (or N/A — explain why)