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 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

@@ -19,6 +19,8 @@ Rationale, Design, TODO, License, Author) if any are still missing.
 
 # Completed Steps
 
+- 2026-10-01: a `DNSWATCHER_DNS_INTERVAL` or `DNSWATCHER_TLS_INTERVAL` that is
+  not a positive duration stops startup instead of being ignored (closes #177).
 - 2026-10-01: wildcard CORS now applies only to the public routes, not to
   `/metrics`, and allows only the methods they serve (closes #100).
 - 2026-10-01: `internal/state` and `internal/watcher` no longer export test-only