Implement the second tier of IRC channel features:
1. Ban system (+b): Add/remove/list bans with wildcard matching.
Bans prevent both joining and sending messages.
Schema: channel_bans table with mask, set_by, created_at.
2. Invite-only (+i): Channel mode requiring invitation to join.
INVITE command for operators. Invites stored in DB and
cleared after successful JOIN.
3. Secret (+s): Hides channel from LIST for non-members and
from WHOIS channel lists when querier is not in same channel.
4. Channel key (+k): Password-protected channels. Key required
on JOIN, set/cleared by operators.
5. User limit (+l): Maximum member count enforcement. Rejects
JOIN when channel is at capacity.
Updated ISUPPORT CHANMODES to b,k,Hl,imnst.
Updated RPL_MYINFO available modes to ikmnostl.
Comprehensive tests for all features at both DB and handler levels.
README updated with full documentation of all new modes.
closes#86
