Enforce channel keys, invites, +s and +H on the IRC listener (closes #122)

The IRC listener's JOIN now passes the channel key on, INVITE records
the invite, LIST and WHOIS hide +s channels from non-members with the
queries the HTTP API uses, and PRIVMSG to a +H channel gets 404.

The INVITE checks and the channel hashcash check move from the HTTP
handlers into the service, so both transports run the same code. An IRC
client cannot attach a stamp, so its PRIVMSG to a +H channel always
fails that check. The unfiltered channel list query, now unused, is
removed.

Model: opus-5-5

internal/db/queries.go

@@ -1482,57 +1482,6 @@ type ChannelInfoFull struct {
 	MemberCount int64  `json:"memberCount"`
 }
 
-// ListAllChannelsWithCounts returns every channel
-// with its member count.
-func (database *Database) ListAllChannelsWithCounts(
-	ctx context.Context,
-) ([]ChannelInfoFull, error) {
-	rows, err := database.conn.QueryContext(ctx,
-		`SELECT c.id, c.name, c.topic,
-		   COUNT(cm.session_id) AS member_count
-		 FROM channels c
-		 LEFT JOIN channel_members cm
-		   ON cm.channel_id = c.id
-		 GROUP BY c.id
-		 ORDER BY c.name`)
-	if err != nil {
-		return nil, fmt.Errorf(
-			"list channels with counts: %w", err,
-		)
-	}
-
-	defer func() { _ = rows.Close() }()
-
-	var out []ChannelInfoFull
-
-	for rows.Next() {
-		var chanInfo ChannelInfoFull
-
-		err = rows.Scan(
-			&chanInfo.ID, &chanInfo.Name,
-			&chanInfo.Topic, &chanInfo.MemberCount,
-		)
-		if err != nil {
-			return nil, fmt.Errorf(
-				"scan channel full: %w", err,
-			)
-		}
-
-		out = append(out, chanInfo)
-	}
-
-	err = rows.Err()
-	if err != nil {
-		return nil, fmt.Errorf("rows error: %w", err)
-	}
-
-	if out == nil {
-		out = []ChannelInfoFull{}
-	}
-
-	return out, nil
-}
-
 // GetChannelCreatedAt returns the creation time of a
 // channel.
 func (database *Database) GetChannelCreatedAt(