Add autocomplete when using /get command

[tarto-dev]

https://docs.disnake.dev/en/latest/ext/commands/slash_commands.html#autocompleters

Suggested by @WegPast

[tarto-dev]

Obviously, suggestions should be related to the server (to prevent confidential-data-leaks and avoid the too big select list).

This point needs to fetch, store and refresh keywords and ensure we avoid memory-leaks (as server cost will stonk)

[github-actions]

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