fix(iroh-dns): lower default TXT TTL from 30s to 5s

[drewr]

This is a dramatic improvement to the service by reconverging a tunnel in ~1 second instead of 30+ seconds.

The TTL is the dominant restart bottleneck: control-plane plumbing (heartbeat → reconcile → DNSRecordSet → Envoy metadata) completes in <1s, but iroh-gateway's resolver serves the stale TXT until it expires.

With this change:

% ./datum-connect tunnel listen --endpoint localhost:9999
Found existing tunnel for localhost:9999:
  id: tunnel-9zllw
  label: 4a66d3990335
  endpoint: http://localhost:9999

Tunnel already configured correctly.

Your endpoint ID: 6b66d57da4dc764e357fc69d454cd101f8a2a7bd60ee682f6f768bc23065b28a
Setting up tunnel...
  ✓ tunnel accepted (0.2s) [HTTPProxy/tunnel-9zllw]
  ✓ TLS certificate issued (0.2s) [HTTPProxy/tunnel-9zllw]
  ✓ connector ready (0.2s) [Connector/datum-connect-mhxj5]
  ✓ iroh DNS published (0.2s) [Connector/datum-connect-mhxj5]
  ✓ route programmed (0.2s) [HTTPProxy/tunnel-9zllw]
  ✓ envoy metadata propagated (1.0s) [HTTPProxy/tunnel-9zllw]
Verifying connectivity...
  ✓ origin reachable (0.1s) [http://localhost:9999]: HTTP 200
  ✓ proxy responding (0.1s) [https://tip-post-nqt7t.datumproxy.net]: HTTP 200
Tunnel ready after 1 sec: https://tip-post-nqt7t.datumproxy.net
Press Ctrl+C to stop...

Measured on datum-connect:

Scenario	Time to proxy responding
Restart immediately after stop, TTL=30	~30s
Restart 35s after stop (TTL elapsed)	~1s

Cost: a few extra authoritative DNS queries per active endpoint. Negligible at connector-zone cardinality.

Refs: datum-cloud/enhancements#756

[scotwells]

@drewr is there documentation on how this is expected to work and why this change resolves the issue? Feels like this is covering an issue somewhere else in the stack.

[drewr]

It centers around the general symptom that a tunnel can take several minutes to provision. Some of that was fixable client-side, but the effect of a long TTL here is reproducible. It shouldn't have downstream effects because nothing else is using Iroh relay TXT records. However, that's not to say that there isn't a better design we could use.

#167