feat(bot): replace fixed activity cycle with weighted random rotation

[karaktaka]

Summary

• Replaces itertools.cycle + fixed @tasks.loop(minutes=5) with a plain asyncio task that owns its own sleep timing
• Activity selection now uses random.choices with weights — "Use / for commands" variants are 3× more likely than flavor entries
• Sleep interval between rotations is randomized between 2–7 minutes (random.uniform(120, 420))
• Adds 4 new flavor activities for more variety

Why asyncio task instead of tasks.loop + change_interval

tasks.loop pre-computes _next_iteration before invoking the callback, so calling change_interval() inside the callback adjusts the iteration after next — not the upcoming sleep. A plain asyncio task with await sleep(...) owns its timing completely with no framework state to fight.

Test plan

• Bot starts and shows "Use / for commands" as the initial activity
• Activity changes every 2–7 minutes (observe over a few cycles)
• "Use / for commands" variants appear noticeably more often than flavor strings
• Bot reconnects cleanly without spawning duplicate activity loops (check on_ready guard)

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