Implement queued cooldown handling for image requests

[dbaldes]

Summary

• add a request queue for image generation commands that enforces the configured cooldown between executions
• deduplicate queued requests with identical prompts and report the remaining wait time to users
• schedule queued executions sequentially, recalculating delays when the cooldown frees up

Testing

• mvn -q -DskipTests package (fails: Could not transfer artifact com.github.pircbotx:pircbotx:pom:2.3.1 from/to jitpack.io (https://jitpack.io): status code: 403, reason phrase: Forbidden (403))

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https://chatgpt.com/codex/tasks/task_e_68e0b9e813048330b795dc32f9b1890a

[dbaldes]

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