200-Feature add megabox quest type with dedicated daily and weekly quest slots

[RemainingDelta]

Changes

• Added quest_category field to DEFAULT_QUESTS and quest documents in MongoDB, enabling separate quest slots per category (message and megabox)
• Updated get_active_quest, assign_random_quest, and update_quest_progress in mongo.py to use a q_key format (daily_message, weekly_megabox, etc.) instead
  of just q_type, so each user always has 4 active quests
• Added two megabox quest definitions:

Quest	Type	Target	Tokens	XP
Mega Box Maniac	Daily	100 opens	50	100
Mega Box Grinder	Weekly	500 opens	250	500

• Added process_quest_update("megabox") call to /megabox and /starrdrop in brawl/commands.py so both commands count toward megabox quest progress
• Added /reset-quests @user admin-only slash command to force-reset a user's quest assignments

Screenshots

2 new megabox quests:

Closes #200