feat: implement Tier 3 utility IRC commands

Implement all 7 utility IRC commands from issue #87:

User commands:
- USERHOST: quick lookup of user@host for up to 5 nicks (RPL 302)
- VERSION: server version string using globals.Version (RPL 351)
- ADMIN: server admin contact info (RPL 256-259)
- INFO: server software info text (RPL 371/374)
- TIME: server local time in RFC format (RPL 391)

Oper commands:
- KILL: forcibly disconnect a user (requires is_oper), broadcasts
  QUIT to all shared channels, cleans up sessions
- WALLOPS: broadcast message to all users with +w usermode
  (requires is_oper)

Supporting changes:
- Add is_wallops column to sessions table in 001_initial.sql
- Add user mode +w tracking via MODE nick +w/-w
- User mode queries now return actual modes (+o, +w)
- MODE -o allows de-opering yourself; MODE +o rejected
- MODE for other users returns ERR_USERSDONTMATCH (502)
- Extract dispatch helpers to reduce dispatchCommand complexity

Tests cover all commands including error cases, oper checks,
user mode set/unset, KILL broadcast, WALLOPS delivery, and
edge cases (self-kill, nonexistent users, missing params).

closes #87

internal/db/queries.go

@@ -2367,3 +2367,132 @@ func (database *Database) SetChannelUserLimit(
 
 	return nil
 }
+
+// SetSessionWallops sets the wallops (+w) flag on a
+// session.
+func (database *Database) SetSessionWallops(
+	ctx context.Context,
+	sessionID int64,
+	enabled bool,
+) error {
+	val := 0
+	if enabled {
+		val = 1
+	}
+
+	_, err := database.conn.ExecContext(
+		ctx,
+		`UPDATE sessions SET is_wallops = ? WHERE id = ?`,
+		val, sessionID,
+	)
+	if err != nil {
+		return fmt.Errorf("set session wallops: %w", err)
+	}
+
+	return nil
+}
+
+// IsSessionWallops returns whether the session has the
+// wallops (+w) usermode set.
+func (database *Database) IsSessionWallops(
+	ctx context.Context,
+	sessionID int64,
+) (bool, error) {
+	var isWallops int
+
+	err := database.conn.QueryRowContext(
+		ctx,
+		`SELECT is_wallops FROM sessions WHERE id = ?`,
+		sessionID,
+	).Scan(&isWallops)
+	if err != nil {
+		return false, fmt.Errorf(
+			"check session wallops: %w", err,
+		)
+	}
+
+	return isWallops != 0, nil
+}
+
+// GetWallopsSessionIDs returns all session IDs that have
+// the wallops (+w) usermode set.
+func (database *Database) GetWallopsSessionIDs(
+	ctx context.Context,
+) ([]int64, error) {
+	rows, err := database.conn.QueryContext(
+		ctx,
+		`SELECT id FROM sessions WHERE is_wallops = 1`,
+	)
+	if err != nil {
+		return nil, fmt.Errorf(
+			"get wallops sessions: %w", err,
+		)
+	}
+
+	defer func() { _ = rows.Close() }()
+
+	var ids []int64
+
+	for rows.Next() {
+		var sessionID int64
+		if scanErr := rows.Scan(&sessionID); scanErr != nil {
+			return nil, fmt.Errorf(
+				"scan wallops session: %w", scanErr,
+			)
+		}
+
+		ids = append(ids, sessionID)
+	}
+
+	if err := rows.Err(); err != nil {
+		return nil, fmt.Errorf(
+			"iterate wallops sessions: %w", err,
+		)
+	}
+
+	return ids, nil
+}
+
+// UserhostInfo holds the data needed for RPL_USERHOST.
+type UserhostInfo struct {
+	Nick        string
+	Username    string
+	Hostname    string
+	IsOper      bool
+	AwayMessage string
+}
+
+// GetUserhostInfo returns USERHOST info for the given
+// nicks. Only nicks that exist are returned.
+func (database *Database) GetUserhostInfo(
+	ctx context.Context,
+	nicks []string,
+) ([]UserhostInfo, error) {
+	if len(nicks) == 0 {
+		return nil, nil
+	}
+
+	results := make([]UserhostInfo, 0, len(nicks))
+
+	for _, nick := range nicks {
+		var info UserhostInfo
+
+		err := database.conn.QueryRowContext(
+			ctx,
+			`SELECT nick, username, hostname,
+			        is_oper, away_message
+			 FROM sessions WHERE nick = ?`,
+			nick,
+		).Scan(
+			&info.Nick, &info.Username, &info.Hostname,
+			&info.IsOper, &info.AwayMessage,
+		)
+		if err != nil {
+			continue // nick not found, skip
+		}
+
+		results = append(results, info)
+	}
+
+	return results, nil
+}