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

README.md

@@ -320,8 +320,8 @@ the following precedence (highest to lowest):
 | `DNSWATCHER_SLACK_WEBHOOK`      | Slack incoming webhook URL                 | `""`        |
 | `DNSWATCHER_MATTERMOST_WEBHOOK` | Mattermost incoming webhook URL            | `""`        |
 | `DNSWATCHER_NTFY_TOPIC`         | ntfy topic URL                             | `""`        |
-| `DNSWATCHER_DNS_INTERVAL`       | DNS check interval                         | `1h`        |
-| `DNSWATCHER_TLS_INTERVAL`       | TLS check interval                         | `12h`       |
+| `DNSWATCHER_DNS_INTERVAL`       | DNS check interval, a positive duration such as `30m`; empty means the default, anything else stops startup | `1h` |
+| `DNSWATCHER_TLS_INTERVAL`       | TLS check interval, a positive duration such as `6h`; empty means the default, anything else stops startup | `12h` |
 | `DNSWATCHER_TLS_EXPIRY_WARNING` | Days before expiry to warn                 | `7`         |
 | `DNSWATCHER_SENTRY_DSN`         | Sentry DSN for error reporting             | `""`        |
 | `DNSWATCHER_MAINTENANCE_MODE`   | Enable maintenance mode                    | `false`     |
@@ -344,6 +344,14 @@ the client address is taken from the `X-Real-IP` or `X-Forwarded-For` header
 the proxy sets; a proxy that sets neither makes all its clients share one
 allowance.
 
+**`DNSWATCHER_DNS_INTERVAL` and `DNSWATCHER_TLS_INTERVAL`** take a positive
+duration: a number followed by a unit such as `s`, `m` or `h`, for example
+`90s`, `30m`, `1h` or `1h30m`. There is no unit for days; write `24h`. An
+unset or empty variable (`DNSWATCHER_DNS_INTERVAL=`) means the default. If
+either is set to anything else, including a bare number or a zero or negative
+duration, dnswatcher refuses to start with an error naming the variable and
+the value.
+
 ### Example `.env`
 
 ```sh