feat: implement Tier 2 channel modes (+b/+i/+s/+k/+l) (#92)

## Summary

Implements the second tier of IRC channel features as described in [#86](#86).

## Features

### 1. Ban System (+b)
- `channel_bans` table with mask, set_by, created_at
- Add/remove/list bans via MODE +b/-b
- Wildcard matching (`*!*@*.example.com`, `badnick!*@*`, etc.)
- Ban enforcement on both JOIN and PRIVMSG
- RPL_BANLIST (367) / RPL_ENDOFBANLIST (368) for ban listing

### 2. Invite-Only (+i)
- `is_invite_only` column on channels table
- INVITE command: operators can invite users
- `channel_invites` table tracks pending invites
- Invites consumed on successful JOIN
- ERR_INVITEONLYCHAN (473) for uninvited JOIN attempts

### 3. Secret (+s)
- `is_secret` column on channels table
- Secret channels hidden from LIST for non-members
- Secret channels hidden from WHOIS channel list for non-members

### 4. Channel Key (+k)
- `channel_key` column on channels table
- MODE +k sets key, MODE -k clears it
- Key required on JOIN (`JOIN #channel key`)
- ERR_BADCHANNELKEY (475) for wrong/missing key

### 5. User Limit (+l)
- `user_limit` column on channels table (0 = no limit)
- MODE +l sets limit, MODE -l removes it
- ERR_CHANNELISFULL (471) when limit reached

## ISUPPORT Changes
- CHANMODES updated to `b,k,Hl,imnst`
- RPL_MYINFO modes updated to `ikmnostl`

## Tests

### Database-level tests:
- Wildcard matching (10 patterns)
- Ban CRUD operations
- Session ban checking
- Invite-only flag toggle
- Invite CRUD + clearing
- Secret channel filtering (LIST and WHOIS)
- Channel key set/get/clear
- User limit set/get/clear

### Handler-level tests:
- Ban add/remove/list via MODE
- Ban blocks JOIN
- Ban blocks PRIVMSG
- Invite-only JOIN rejection + INVITE acceptance
- Secret channel hidden from LIST
- Channel key required on JOIN
- User limit enforcement
- Mode string includes new modes
- ISUPPORT updated CHANMODES
- Non-operators cannot set any Tier 2 modes

## Schema Changes
- Added `is_invite_only`, `is_secret`, `channel_key`, `user_limit` to `channels` table
- Added `channel_bans` table
- Added `channel_invites` table
- All changes in `001_initial.sql` (pre-1.0.0 repo)

closes #86

Co-authored-by: user <user@Mac.lan guest wan>
Reviewed-on: #92
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

README.md

@@ -1079,8 +1079,8 @@ the server to the client (never C2S) and use 3-digit string codes in the
 | `001` | RPL_WELCOME         | After session creation | `{"command":"001","to":"alice","body":["Welcome to the network, alice"]}` |
 | `002` | RPL_YOURHOST        | After session creation | `{"command":"002","to":"alice","body":["Your host is neoirc, running version 0.1"]}` |
 | `003` | RPL_CREATED         | After session creation | `{"command":"003","to":"alice","body":["This server was created 2026-02-10"]}` |
-| `004` | RPL_MYINFO          | After session creation | `{"command":"004","to":"alice","params":["neoirc","0.1","","mnst"]}` |
-| `005` | RPL_ISUPPORT        | After session creation | `{"command":"005","to":"alice","params":["CHANTYPES=#","NICKLEN=32","PREFIX=(ov)@+","CHANMODES=,,H,mnst","NETWORK=neoirc"],"body":["are supported by this server"]}` |
+| `004` | RPL_MYINFO          | After session creation | `{"command":"004","to":"alice","params":["neoirc","0.1","","ikmnostl"]}` |
+| `005` | RPL_ISUPPORT        | After session creation | `{"command":"005","to":"alice","params":["CHANTYPES=#","NICKLEN=32","PREFIX=(ov)@+","CHANMODES=b,k,Hl,imnst","NETWORK=neoirc"],"body":["are supported by this server"]}` |
 | `221` | RPL_UMODEIS         | In response to user MODE query | `{"command":"221","to":"alice","body":["+"]}` |
 | `251` | RPL_LUSERCLIENT     | On connect or LUSERS command | `{"command":"251","to":"alice","body":["There are 5 users and 0 invisible on 1 servers"]}` |
 | `252` | RPL_LUSEROP         | On connect or LUSERS command | `{"command":"252","to":"alice","params":["0"],"body":["operator(s) online"]}` |
@@ -1128,12 +1128,15 @@ Inspired by IRC, simplified:
 
 | Mode | Name           | Meaning | Status |
 |------|----------------|---------|--------|
+| `+b` | Ban            | Prevents matching hostmasks from joining or sending (parameter: `nick!user@host` mask with wildcards) | **Enforced** |
+| `+i` | Invite-only    | Only invited users can join; use `INVITE nick #channel` to invite | **Enforced** |
+| `+k` | Channel key    | Requires a password to join (parameter: key string) | **Enforced** |
+| `+l` | User limit     | Maximum number of members allowed in the channel (parameter: integer) | **Enforced** |
 | `+m` | Moderated      | Only voiced (`+v`) users and operators (`+o`) can send | **Enforced** |
-| `+t` | Topic lock     | Only operators can change the topic (default: ON) | **Enforced** |
 | `+n` | No external    | Only channel members can send messages to the channel | **Enforced** |
+| `+s` | Secret         | Channel hidden from LIST and WHOIS for non-members | **Enforced** |
+| `+t` | Topic lock     | Only operators can change the topic (default: ON) | **Enforced** |
 | `+H` | Hashcash       | Requires proof-of-work for PRIVMSG (parameter: bits, e.g. `+H 20`) | **Enforced** |
-| `+i` | Invite-only    | Only invited users can join | Not yet enforced |
-| `+s` | Secret         | Channel hidden from LIST response | Not yet enforced |
 
 **User channel modes (set per-user per-channel):**
 
@@ -1145,6 +1148,42 @@ Inspired by IRC, simplified:
 **Channel creator auto-op:** The first user to JOIN a channel (creating it)
 automatically receives `+o` operator status.
 
+**Ban system (+b):** Operators can ban users by hostmask pattern with wildcard
+matching (`*` and `?`). `MODE #channel +b` with no argument lists current bans.
+Bans prevent both joining and sending messages.
+
+```
+MODE #channel +b *!*@*.example.com   — ban all users from example.com
+MODE #channel -b *!*@*.example.com   — remove the ban
+MODE #channel +b                     — list all bans (RPL_BANLIST 367/368)
+```
+
+**Invite-only (+i):** When set, users must be invited by an operator before
+joining. The `INVITE` command records an invite that is consumed on JOIN.
+
+```
+MODE #channel +i                     — set invite-only
+INVITE nick #channel                 — invite a user (operator only on +i channels)
+```
+
+**Channel key (+k):** Requires a password to join the channel.
+
+```
+MODE #channel +k secretpass          — set a channel key
+MODE #channel -k *                   — remove the key
+JOIN #channel secretpass             — join with key
+```
+
+**User limit (+l):** Caps the number of members in the channel.
+
+```
+MODE #channel +l 50                  — set limit to 50 members
+MODE #channel -l                     — remove the limit
+```
+
+**Secret (+s):** Hides the channel from `LIST` for non-members and from
+`WHOIS` channel lists when the querier is not in the same channel.
+
 **KICK command:** Channel operators can remove users with `KICK #channel nick
 [:reason]`. The kicked user and all channel members receive the KICK message.
 
@@ -1153,7 +1192,7 @@ RPL_AWAY), and skips hashcash validation on +H channels (servers and services
 use NOTICE).
 
 **ISUPPORT:** The server advertises `PREFIX=(ov)@+` and
-`CHANMODES=,,H,mnst` in RPL_ISUPPORT (005).
+`CHANMODES=b,k,Hl,imnst` in RPL_ISUPPORT (005).
 
 ### Per-Channel Hashcash (Anti-Spam)
 
@@ -2695,7 +2734,7 @@ guess is borne by the server (bcrypt), not the client.
 - [x] **Client output queue pruning** — delete old client output queue entries per `QUEUE_MAX_AGE`
 - [x] **Message rotation** — prune messages older than `MESSAGE_MAX_AGE`
 - [x] **Channel modes** — enforce `+m` (moderated), `+t` (topic lock), `+n` (no external)
-- [ ] **Channel modes (tier 2)** — enforce `+i` (invite-only), `+s` (secret), `+b` (ban), `+k` (key), `+l` (limit)
+- [x] **Channel modes (tier 2)** — enforce `+i` (invite-only), `+s` (secret), `+b` (ban), `+k` (key), `+l` (limit)
 - [x] **User channel modes** — `+o` (operator), `+v` (voice) with NAMES prefixes
 - [x] **KICK command** — operator-only channel kick with broadcast
 - [x] **MODE command** — query and set channel/user modes