refactor: replace HTTP error codes with IRC numeric replies for all IRC commands

IRC commands (PRIVMSG, JOIN, PART, NICK, TOPIC, etc.) now respond with
proper IRC numeric replies delivered through the message queue instead of
HTTP status codes. HTTP error codes are now reserved exclusively for
transport-level concerns: auth failures (401), malformed requests (400),
and server errors (500).

Changes:
- Add params column to messages table for IRC-style parameters
- Add Params field to IRCMessage struct and update all queries
- Add respondIRCError helper for consistent IRC error delivery
- Add RPL_WELCOME (001) on session creation and login
- Add RPL_TOPIC/RPL_NOTOPIC (332/331), RPL_NAMREPLY (353),
  RPL_ENDOFNAMES (366) on JOIN
- Add RPL_TOPIC (332) on TOPIC set
- Replace HTTP 404 with ERR_NOSUCHCHANNEL (403) and ERR_NOSUCHNICK (401)
- Replace HTTP 409 with ERR_NICKNAMEINUSE (433)
- Replace HTTP 403 with ERR_NOTONCHANNEL (442)
- Replace HTTP 400 with ERR_NEEDMOREPARAMS (461), ERR_ERRONEUSNICKNAME (432),
  and ERR_UNKNOWNCOMMAND (421) where appropriate
- Change PRIVMSG/NOTICE success from HTTP 201 to HTTP 200
- Update all tests to verify IRC numerics in message queue
- Add new tests for RPL_WELCOME and JOIN numerics
- Update README to document new numeric reply behavior

closes #54

internal/db/queries.go

@@ -35,6 +35,7 @@ type IRCMessage struct {
 	Command string          `json:"command"`
 	From    string          `json:"from,omitempty"`
 	To      string          `json:"to,omitempty"`
+	Params  json.RawMessage `json:"params,omitempty"`
 	Body    json.RawMessage `json:"body,omitempty"`
 	TS      string          `json:"ts"`
 	Meta    json.RawMessage `json:"meta,omitempty"`
@@ -491,12 +492,17 @@ func (database *Database) GetSessionChannelIDs(
 func (database *Database) InsertMessage(
 	ctx context.Context,
 	command, from, target string,
+	params json.RawMessage,
 	body json.RawMessage,
 	meta json.RawMessage,
 ) (int64, string, error) {
 	msgUUID := uuid.New().String()
 	now := time.Now().UTC()
 
+	if params == nil {
+		params = json.RawMessage("[]")
+	}
+
 	if body == nil {
 		body = json.RawMessage("[]")
 	}
@@ -508,10 +514,10 @@ func (database *Database) InsertMessage(
 	res, err := database.conn.ExecContext(ctx,
 		`INSERT INTO messages
 		 (uuid, command, msg_from, msg_to,
-		  body, meta, created_at)
-		 VALUES (?, ?, ?, ?, ?, ?, ?)`,
+		  params, body, meta, created_at)
+		 VALUES (?, ?, ?, ?, ?, ?, ?, ?)`,
 		msgUUID, command, from, target,
-		string(body), string(meta), now)
+		string(params), string(body), string(meta), now)
 	if err != nil {
 		return 0, "", fmt.Errorf(
 			"insert message: %w", err,
@@ -578,7 +584,7 @@ func (database *Database) PollMessages(
 	rows, err := database.conn.QueryContext(ctx,
 		`SELECT cq.id, m.uuid, m.command,
 		   m.msg_from, m.msg_to,
-		   m.body, m.meta, m.created_at
+		   m.params, m.body, m.meta, m.created_at
 		 FROM client_queues cq
 		 INNER JOIN messages m
 		   ON m.id = cq.message_id
@@ -642,7 +648,7 @@ func (database *Database) queryHistory(
 	if beforeID > 0 {
 		rows, err := database.conn.QueryContext(ctx,
 			`SELECT id, uuid, command, msg_from,
-			   msg_to, body, meta, created_at
+			   msg_to, params, body, meta, created_at
 			 FROM messages
 			 WHERE msg_to = ? AND id < ?
 			   AND command = 'PRIVMSG'
@@ -659,7 +665,7 @@ func (database *Database) queryHistory(
 
 	rows, err := database.conn.QueryContext(ctx,
 		`SELECT id, uuid, command, msg_from,
-		   msg_to, body, meta, created_at
+		   msg_to, params, body, meta, created_at
 		 FROM messages
 		 WHERE msg_to = ?
 		   AND command = 'PRIVMSG'
@@ -684,16 +690,16 @@ func scanMessages(
 
 	for rows.Next() {
 		var (
-			msg        IRCMessage
-			qID        int64
-			body, meta string
-			createdAt  time.Time
+			msg                IRCMessage
+			qID                int64
+			params, body, meta string
+			createdAt          time.Time
 		)
 
 		err := rows.Scan(
 			&qID, &msg.ID, &msg.Command,
 			&msg.From, &msg.To,
-			&body, &meta, &createdAt,
+			&params, &body, &meta, &createdAt,
 		)
 		if err != nil {
 			return nil, fallbackQID, fmt.Errorf(
@@ -701,6 +707,10 @@ func scanMessages(
 			)
 		}
 
+		if params != "" && params != "[]" {
+			msg.Params = json.RawMessage(params)
+		}
+
 		msg.Body = json.RawMessage(body)
 		msg.Meta = json.RawMessage(meta)
 		msg.TS = createdAt.Format(time.RFC3339Nano)