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>

Dockerfile

@@ -53,7 +53,7 @@ RUN apk add --no-cache ca-certificates \
 COPY --from=builder /neoircd /usr/local/bin/neoircd
 
 USER neoirc
-EXPOSE 8080
+EXPOSE 8080 6667
 HEALTHCHECK --interval=30s --timeout=3s --start-period=5s --retries=3 \
     CMD wget -qO- http://localhost:8080/.well-known/healthcheck.json || exit 1
 ENTRYPOINT ["neoircd"]