Feat/cross channel spam automod

[Neil-Tomar]

Summary

Adds a second spam automod — checkCrossChannelSpam in AutoMod — that catches a pattern the existing same-channel spam check misses: a single user rapidly spreading messages across several different channels.

What it does

• Flags a user who posts in more than a configured number of distinct channels within a configured time window.
• On a match, applies the existing 6-hour spam timeout and deletes the matched messages.
• Both thresholds live in ModerationConfig, so they're tunable per guild.

How it works

• CachedMessage now stores a channelId, taken from the live message when it's cached — so the cache knows which channel each message came from. That's what lets the check count distinct channels without any extra Discord API calls; only the timeout and the deletions talk to Discord.
• checkCrossChannelSpam reads the author's recent messages from the MessageCache within the window, gathers the distinct channels, and passes the messages to handleSpam once the count crosses the threshold.
• handleSpam gains a List<CachedMessage> overload that times the user out and deletes each message by id.
• New ModerationConfig fields: crossChannelSpamWindowSeconds and crossChannelSpamMinChannels.

Notes

• channelId isn't persisted to the database yet — it defaults to -1 when read from the DB, per your suggestion. If it holds up, I can follow with a migration to store it properly.
• The trigger currently uses the distinct-channel count only. Your spec also mentioned a message count, so I'm happy to add a separate minimum-messages threshold if you'd like both.