feat: add irc numerics package, deduplicate constants, fix dead code

- Create internal/irc/ package with all IRC numeric reply codes (RFC 1459/2812)
  and command string constants as the single source of truth
- Replace all 69+ bare numeric string literals in api.go with named constants
  (e.g. irc.RplWelcome, irc.ErrNoSuchChannel)
- Add 'code' (int) and named 'command' (e.g. RPL_YOURHOST) fields to IRC
  message JSON replies via irc.Name() lookup in scanMessages
- Deduplicate command constants: remove local definitions from api.go,
  cmd/neoirc-cli/main.go, and cmd/neoirc-cli/api/client.go; all now import
  from internal/irc
- Fix dead code: remove handleListCmd/handleWhoCmd/handleWhoisCmd/
  sendWhoisNumerics that were unreachable due to dispatchCommand routing
  LIST/WHO/WHOIS to dispatchInfoCommand before dispatchQueryCommand.
  Route these commands to dispatchQueryCommand which has the improved
  implementations (e.g. ListAllChannelsWithCounts single-query vs N+1)
- Update enqueueNumeric and respondIRCError signatures from string to int
- Update test helper findNumeric to check the new 'code' JSON field

Closes #52

internal/db/queries.go

@@ -7,8 +7,10 @@ import (
 	"encoding/hex"
 	"encoding/json"
 	"fmt"
+	"strconv"
 	"time"
 
+	"git.eeqj.de/sneak/neoirc/internal/irc"
 	"github.com/google/uuid"
 )
 
@@ -33,6 +35,7 @@ func generateToken() (string, error) {
 type IRCMessage struct {
 	ID      string          `json:"id"`
 	Command string          `json:"command"`
+	Code    int             `json:"code,omitempty"`
 	From    string          `json:"from,omitempty"`
 	To      string          `json:"to,omitempty"`
 	Params  json.RawMessage `json:"params,omitempty"`
@@ -42,6 +45,15 @@ type IRCMessage struct {
 	DBID    int64           `json:"-"`
 }
 
+// isNumericCode returns true if s is exactly a 3-digit
+// IRC numeric reply code.
+func isNumericCode(s string) bool {
+	return len(s) == 3 &&
+		s[0] >= '0' && s[0] <= '9' &&
+		s[1] >= '0' && s[1] <= '9' &&
+		s[2] >= '0' && s[2] <= '9'
+}
+
 // ChannelInfo is a lightweight channel representation.
 type ChannelInfo struct {
 	ID    int64  `json:"id"`
@@ -717,6 +729,15 @@ func scanMessages(
 		msg.DBID = qID
 		lastQID = qID
 
+		if isNumericCode(msg.Command) {
+			code, _ := strconv.Atoi(msg.Command)
+			msg.Code = code
+
+			if name := irc.Name(code); name != "" {
+				msg.Command = name
+			}
+		}
+
 		msgs = append(msgs, msg)
 	}