config: stop startup on an invalid DNS or TLS interval (closes #177)

DNSWATCHER_DNS_INTERVAL and DNSWATCHER_TLS_INTERVAL were parsed with
time.ParseDuration and silently replaced by the default when that failed,
so a value like 5 or 1d gave hourly checks with no hint why, and zero or
negative values were accepted. Both now go through parseInterval, which
returns an error naming the variable and the value, and startup stops the
same way it does for invalid targets. An unset or empty variable still gets
its default from setupViper. The three tests that pinned the old fallback
are replaced. The README says what a valid value looks like.

Model: opus-5-5

TODO.md

@@ -20,6 +20,8 @@ https://git.eeqj.de/sneak/dnswatcher/issues/104
 
 # Completed Steps
 
+- 2026-10-01: a `DNSWATCHER_DNS_INTERVAL` or `DNSWATCHER_TLS_INTERVAL` that is
+  not a positive duration stops startup; empty means the default (closes #177).
 - 2026-10-01: `/metrics` allows each client address 30 requests a minute,
   counted before Basic Auth, and answers 429 beyond that (closes #101).
 - 2026-10-01: the image built by `make docker` reports the `git describe`
@@ -97,8 +99,6 @@ https://git.eeqj.de/sneak/dnswatcher/issues/104
 - nameserver IP address changes: https://git.eeqj.de/sneak/dnswatcher/issues/105
 - `DNSWATCHER_SENTRY_DSN` does nothing:
   https://git.eeqj.de/sneak/dnswatcher/issues/107
-- invalid DNS or TLS interval silently replaced by the default:
-  https://git.eeqj.de/sneak/dnswatcher/issues/177
 - trial run of the finished image:
   https://git.eeqj.de/sneak/dnswatcher/issues/149
 - 1.0 readiness: run it with a real config and read the logs: