Add per-guild locking to prevent race conditions in concurrent config access

[Claude]

Multiple cogs had race conditions where concurrent async operations could overwrite each other's config changes. When two tasks read → modify → write simultaneously, the second write would silently drop the first task's changes.

Changes

party/party.py

• Add per-guild locks protecting party creation, editing, and user signup operations

secret_santa/secret_santa.py

• Add per-guild locks protecting wishlist updates and event modifications

quotesdb/quotedb.py

• Add per-guild locks protecting quote addition and deletion operations

assign_roles/assign_roles.py

• Add per-guild locks protecting role authorization/deauthorization
• Fix missing duplicate check: two concurrent authorizations could both pass the initial check and append duplicate entries

Implementation

Each cog now maintains a per-guild lock dictionary, following the pattern already used in albion_bandits:

def __init__(self, bot):
    self._config_locks = {}  # {guild_id: asyncio.Lock}
    
def _get_guild_lock(self, guild_id: int) -> asyncio.Lock:
    if guild_id not in self._config_locks:
        self._config_locks[guild_id] = asyncio.Lock()
    return self._config_locks[guild_id]

# Critical sections wrapped with lock before config context manager
async def operation(self, ctx, ...):
    lock = self._get_guild_lock(ctx.guild.id)
    async with lock:
        async with self.config.guild(ctx.guild).data() as data:
            # modify data

Locks are per-guild to avoid blocking operations across different servers. Each cog's config is modified independently under lock protection.