refactor: split db module, extract dashboard to CF Pages, upgrade zat
- split db/mod.zig (527→110 lines) into activity, search, stats, write
- extract dashboard HTML/CSS/JS to site/ for Cloudflare Pages
- add /api/dashboard JSON endpoint, /dashboard redirects to CF Pages
- upgrade zat to 0.1.0 (tangled.sh), use CommitAction enum in tap.zig
- link stack section in README to bsky post
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>