feat: implement Tier 2 channel modes (+b/+i/+s/+k/+l)

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

internal/db/schema/001_initial.sql

@@ -42,10 +42,36 @@ CREATE TABLE IF NOT EXISTS channels (
     hashcash_bits INTEGER NOT NULL DEFAULT 0,
     is_moderated INTEGER NOT NULL DEFAULT 0,
     is_topic_locked INTEGER NOT NULL DEFAULT 1,
+    is_invite_only INTEGER NOT NULL DEFAULT 0,
+    is_secret INTEGER NOT NULL DEFAULT 0,
+    channel_key TEXT NOT NULL DEFAULT '',
+    user_limit INTEGER NOT NULL DEFAULT 0,
     created_at DATETIME DEFAULT CURRENT_TIMESTAMP,
     updated_at DATETIME DEFAULT CURRENT_TIMESTAMP
 );
 
+-- Channel bans
+CREATE TABLE IF NOT EXISTS channel_bans (
+    id INTEGER PRIMARY KEY AUTOINCREMENT,
+    channel_id INTEGER NOT NULL REFERENCES channels(id) ON DELETE CASCADE,
+    mask TEXT NOT NULL,
+    set_by TEXT NOT NULL,
+    created_at DATETIME DEFAULT CURRENT_TIMESTAMP,
+    UNIQUE(channel_id, mask)
+);
+CREATE INDEX IF NOT EXISTS idx_channel_bans_channel ON channel_bans(channel_id);
+
+-- Channel invites (in-memory would be simpler but DB survives restarts)
+CREATE TABLE IF NOT EXISTS channel_invites (
+    id INTEGER PRIMARY KEY AUTOINCREMENT,
+    channel_id INTEGER NOT NULL REFERENCES channels(id) ON DELETE CASCADE,
+    session_id INTEGER NOT NULL REFERENCES sessions(id) ON DELETE CASCADE,
+    invited_by TEXT NOT NULL DEFAULT '',
+    created_at DATETIME DEFAULT CURRENT_TIMESTAMP,
+    UNIQUE(channel_id, session_id)
+);
+CREATE INDEX IF NOT EXISTS idx_channel_invites_channel ON channel_invites(channel_id);
+
 -- Channel members
 CREATE TABLE IF NOT EXISTS channel_members (
     id INTEGER PRIMARY KEY AUTOINCREMENT,