Fix fly-deploy WARNING by starting nginx before Tailscale (#128)

## Summary
- Start nginx before Tailscale in `start.sh` so port 8080 is bound immediately, eliminating the "app is not listening on the expected address" WARNING during `fly deploy`
- Switch `proxy_pass` to use a variable with `resolver 100.100.100.100 valid=30s` so nginx can start without resolving MagicDNS names at config load time
- DNS results cached 30s per worker — no per-request lookup overhead

## Context
The WARNING was a race condition: Fly checks for listeners right after the machine starts, but `start.sh` ran ~5-10s of Tailscale setup before starting nginx. The health check always passed later, but the warning was noisy.

## Test plan
- [ ] Merge and let the deploy-fly workflow trigger
- [ ] Check runner logs for absence of the WARNING
- [ ] Verify `docs.eblu.me` still serves correctly
- [ ] Verify `/healthz` still passes

Reviewed-on: https://forge.ops.eblu.me/eblume/blumeops/pulls/128

docs/changelog.d/fix-fly-deploy-warnings.bugfix.md

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+Fix fly-deploy WARNING by starting nginx before Tailscale, deferring upstream DNS resolution to request time.