fix: blacklist CitronusAcademy/traefik-maintenance-plugin

[mmatur]

Summary

• Blacklists CitronusAcademy/traefik-maintenance-plugin to prevent piceus from panicking in production
• The plugin's .traefik.yml has empty string map keys ("") in testData, which causes a panic in paerser's isArrayKey function ([]rune(key)[0] with empty string → index out of range)

Root cause

testData:
  environmentEndpoints:
    "": "https://..."   # empty key
  environmentSecrets:
    "":                 # empty key
      header: "..."