[feat]: implement SpamProcessor and /spammonitor list command

[not-meet]

Overview

Follow-up to #3 The detection foundation (data model, cache, persistence)
is in place. This issue tracks wiring it into an actual processor and
exposing the first admin command to query flagged users.

Tasks

• Implement SpamProcessor with all 6 detection gates
  (exact duplicate, fuzzy/polymorphic, cross-channel, rate flood, room spread, URL spam)
• Add getAll(read) to UserStatusStore to fetch all flagged users
• Implement /spammonitor list slash command

Expected behaviour

/spammonitor list returns a reply to the invoking admin
listing all users with spammingLevel > Clean, sorted by severity.