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>

internal/db/testing.go

@@ -0,0 +1,25 @@
+package db
+
+import (
+	"context"
+	"database/sql"
+	"log/slog"
+)
+
+// NewTestDatabaseFromConn creates a Database wrapping an
+// existing *sql.DB connection.  Intended for integration
+// tests in other packages.
+func NewTestDatabaseFromConn(conn *sql.DB) *Database {
+	return &Database{ //nolint:exhaustruct
+		conn: conn,
+		log:  slog.Default(),
+	}
+}
+
+// RunMigrations applies all schema migrations.  Exposed
+// for integration tests in other packages.
+func (database *Database) RunMigrations(
+	ctx context.Context,
+) error {
+	return database.runMigrations(ctx)
+}