config: a bad number or true/false setting stops startup (closes #269)

PORT and DNSWATCHER_TLS_EXPIRY_WARNING were read with viper.GetInt,
and DNSWATCHER_DEBUG, DNSWATCHER_MAINTENANCE_MODE and
DNSWATCHER_SEND_TEST_NOTIFICATION with viper.GetBool, which turn a
value that does not parse into 0 or false with no error. Each is now
parsed the way parseInterval parses an interval: PORT must be 1 to
65535, the expiry warning 1 or more days, the true/false settings a
value strconv.ParseBool accepts. Anything else stops startup with an
error naming the variable and the value; empty still means the
default.

Reading the config file moved, unchanged, from buildConfig into
readConfigFile, so buildConfig stays within the linter's length limit.

Model: opus-5-5

TODO.md

@@ -19,6 +19,8 @@ trial run of the finished image: https://git.eeqj.de/sneak/dnswatcher/issues/149
 
 # Completed Steps
 
+- 2026-10-13: a bad `PORT`, `DNSWATCHER_TLS_EXPIRY_WARNING` or true/false
+  setting stops startup instead of being read as 0 or false (closes #269).
 - 2026-10-13: a check saving the state while the dashboard or `/api/v1/status`
   is read cannot crash the process (closes #261).
 - 2026-10-06: canonical files re-fetched from `sneak/prompts` at `dd4027b`; lint