feat: add IRC wire protocol listener with shared service layer

Add a traditional IRC wire protocol listener (RFC 1459/2812) on
configurable port (default :6667), sharing business logic with
the HTTP API via a new service layer.

- IRC listener: NICK, USER, PASS, JOIN, PART, PRIVMSG, NOTICE,
  TOPIC, MODE, KICK, QUIT, NAMES, LIST, WHOIS, WHO, AWAY, OPER,
  INVITE, LUSERS, MOTD, PING/PONG, CAP
- Service layer: shared logic for both transports including
  channel join (with Tier 2 checks: ban/invite/key/limit),
  message send (with ban + moderation checks), nick change,
  topic, kick, mode, quit broadcast, away, oper, invite
- BroadcastQuit uses FanOut pattern (one insert, N enqueues)
- HTTP handlers delegate to service for all command logic
- Tier 2 mode operations (+b/+i/+s/+k/+l) use service methods

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"]