Add configurable per-user command cooldown

[Saksham-official]

This PR adds a default per-user cooldown system to prevent command spamming, addressing #29.

What was done:

• Added CooldownManager in utils/cooldown.py to track timestamps per user, calculate retry intervals, and prune old entries if the cache grows large.
• Added COMMAND_COOLDOWN_SECONDS to Config (defaults to 3.0s, documented in .env.example). Setting it to 0 or negative disables the cooldown.
• Added global checks for prefix commands (@self.check) and slash commands (tree.interaction_check, scoped to application commands so autocomplete and buttons aren't affected).
• Commands that define their own custom cooldowns (like ?fridge in fun.py or tts.py) keep their existing rate limits and are skipped by the default gate.
• Bot owner (OWNER_ID) bypasses the cooldown.
• Connected self.tree.on_error = self.on_app_command_error so slash cooldown errors properly trigger the existing ephemeral response handler.
• Added unit tests in tests/test_cooldown.py.

Verified locally with pytest, ruff check ., and mypy ..

Closes #29