feat: add username/hostname support with IRC hostmask format

- Add username and hostname columns to sessions table (001_initial.sql)
- Accept optional username field in session creation and registration
  endpoints; defaults to nick if not provided
- Resolve hostname via reverse DNS of connecting client IP at session
  creation time (supports X-Forwarded-For and X-Real-IP headers)
- Display real username and hostname in WHOIS (311 RPL_WHOISUSER) and
  WHO (352 RPL_WHOREPLY) responses instead of nick/servername
- Add FormatHostmask helper for nick!user@host format
- Add SessionHostInfo type and GetSessionHostInfo query
- Include username/hostname in MemberInfo and ChannelMembers results
- Extract validateHashcash and resolveUsername helpers to stay under
  funlen limits
- Add comprehensive unit tests for all new DB functions, hostmask
  formatting, and integration tests for WHOIS/WHO responses
- Update README with hostmask documentation, new API fields, and
  updated schema reference

internal/db/queries.go

@@ -74,14 +74,40 @@ type ChannelInfo struct {
 type MemberInfo struct {
 	ID       int64     `json:"id"`
 	Nick     string    `json:"nick"`
+	Username string    `json:"username"`
+	Hostname string    `json:"hostname"`
 	LastSeen time.Time `json:"lastSeen"`
 }
 
+// Hostmask returns the IRC hostmask in
+// nick!user@host format.
+func (m *MemberInfo) Hostmask() string {
+	return FormatHostmask(m.Nick, m.Username, m.Hostname)
+}
+
+// FormatHostmask formats a nick, username, and hostname
+// into a standard IRC hostmask string (nick!user@host).
+func FormatHostmask(nick, username, hostname string) string {
+	if username == "" {
+		username = nick
+	}
+
+	if hostname == "" {
+		hostname = "*"
+	}
+
+	return nick + "!" + username + "@" + hostname
+}
+
 // CreateSession registers a new session and its first client.
 func (database *Database) CreateSession(
 	ctx context.Context,
-	nick string,
+	nick, username, hostname string,
 ) (int64, int64, string, error) {
+	if username == "" {
+		username = nick
+	}
+
 	sessionUUID := uuid.New().String()
 	clientUUID := uuid.New().String()
 
@@ -101,9 +127,10 @@ func (database *Database) CreateSession(
 
 	res, err := transaction.ExecContext(ctx,
 		`INSERT INTO sessions
-		 (uuid, nick, created_at, last_seen)
-		 VALUES (?, ?, ?, ?)`,
-		sessionUUID, nick, now, now)
+		 (uuid, nick, username, hostname,
+		  created_at, last_seen)
+		 VALUES (?, ?, ?, ?, ?, ?)`,
+		sessionUUID, nick, username, hostname, now, now)
 	if err != nil {
 		_ = transaction.Rollback()
 
@@ -209,6 +236,35 @@ func (database *Database) GetSessionByNick(
 	return sessionID, nil
 }
 
+// SessionHostInfo holds the username and hostname for a session.
+type SessionHostInfo struct {
+	Username string
+	Hostname string
+}
+
+// GetSessionHostInfo returns the username and hostname
+// for a session.
+func (database *Database) GetSessionHostInfo(
+	ctx context.Context,
+	sessionID int64,
+) (*SessionHostInfo, error) {
+	var info SessionHostInfo
+
+	err := database.conn.QueryRowContext(
+		ctx,
+		`SELECT username, hostname
+		 FROM sessions WHERE id = ?`,
+		sessionID,
+	).Scan(&info.Username, &info.Hostname)
+	if err != nil {
+		return nil, fmt.Errorf(
+			"get session host info: %w", err,
+		)
+	}
+
+	return &info, nil
+}
+
 // GetChannelByName returns the channel ID for a name.
 func (database *Database) GetChannelByName(
 	ctx context.Context,
@@ -388,7 +444,8 @@ func (database *Database) ChannelMembers(
 	channelID int64,
 ) ([]MemberInfo, error) {
 	rows, err := database.conn.QueryContext(ctx,
-		`SELECT s.id, s.nick, s.last_seen
+		`SELECT s.id, s.nick, s.username,
+		   s.hostname, s.last_seen
 		 FROM sessions s
 		 INNER JOIN channel_members cm
 		   ON cm.session_id = s.id
@@ -408,7 +465,9 @@ func (database *Database) ChannelMembers(
 		var member MemberInfo
 
 		err = rows.Scan(
-			&member.ID, &member.Nick, &member.LastSeen,
+			&member.ID, &member.Nick,
+			&member.Username, &member.Hostname,
+			&member.LastSeen,
 		)
 		if err != nil {
 			return nil, fmt.Errorf(