feat: Reset manual topic changes in incident channels

[spalmurray]

Adds a Slack event handler that reverts manual topic edits in incident channels back to the Firetower-managed topic and sends the editor an ephemeral explaining the reset, quoting what they tried to set, and pointing them to /ft subject, /ft severity, and /ft captain to change incident details instead. This is firetower's first Socket Mode event subscription (message.channels/message.groups), so a no-op catch-all suppresses unhandled-message log spam; a bot-author guard prevents reset loops; and an in-memory event-id cache dedups Slack redeliveries (correct as long as the firetower-slack-app service stays single-instance). User-supplied topic text is escaped before it goes into the ephemeral.

Tested in test env. Just need to update prod manifest for new events.

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