refactor: migrate HTTP handlers to shared service layer

- Migrate all HTTP command handlers (PRIVMSG, JOIN, PART, NICK, TOPIC,
  KICK, QUIT, AWAY, OPER, MODE) to use hdlr.svc.* service methods
  instead of direct database calls. Both HTTP and IRC transports now
  share the same business logic path.

- Fix BroadcastQuit bug: was inserting N separate message rows (one per
  recipient); now uses FanOut pattern with 1 InsertMessage + N
  EnqueueToSession calls.

- Fix README: IRC listener is enabled by default on :6667, not
  disabled. Remove redundant -e IRC_LISTEN_ADDR from Docker example.

- Add EXPOSE 6667 to Dockerfile alongside existing HTTP port.

- Add service layer unit tests (JoinChannel, PartChannel,
  SendChannelMessage, FanOut, BroadcastQuit, moderated channel).

- Update handler test setup to provide Service instance.

- Use constant-time comparison in Oper credential validation to
  prevent timing attacks.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

README.md

@@ -2252,17 +2252,15 @@ neoirc includes an optional traditional IRC wire protocol listener (RFC
 backward compatibility with existing IRC clients like irssi, weechat, hexchat,
 and others.
 
-### Enabling
+### Configuration
 
-Set the `IRC_LISTEN_ADDR` environment variable to a TCP address:
+The IRC listener is **enabled by default** on `:6667`. To disable it, set
+`IRC_LISTEN_ADDR` to an empty string:
 
 ```bash
-IRC_LISTEN_ADDR=:6667
+IRC_LISTEN_ADDR=
 ```
 
-When unset or empty, the IRC listener is disabled and only the HTTP/JSON API is
-available.
-
 ### Supported Commands
 
 | Category   | Commands                                             |
@@ -2297,13 +2295,13 @@ connected via the HTTP API can communicate in the same channels seamlessly.
 
 ### Docker Usage
 
-To expose the IRC port in Docker:
+To expose the IRC port in Docker (the listener is enabled by default on
+`:6667`):
 
 ```bash
 docker run -d \
   -p 8080:8080 \
   -p 6667:6667 \
-  -e IRC_LISTEN_ADDR=:6667 \
   -v neoirc-data:/var/lib/neoirc \
   neoirc
 ```