Complete IRC numerics module and move to pkg/irc/ (refs #52) (#71)

This PR addresses [issue #52](#52):

- **Adds all missing numeric reply codes** from RFC 1459 and RFC 2812, making the module spec-complete
- **Moves the package** from `internal/irc/` to `pkg/irc/` to indicate external usefulness
- **Updates all imports** throughout the codebase

### Added numerics

- Trace replies (200-209)
- Stats replies (211-219, 242-243)
- Service replies (234-235)
- Admin replies (256-259)
- Trace log/end, try again (261-263)
- WHOWAS (314, 369)
- List start (321), unique ops (325)
- Invite/except lists (346-349)
- Version (351), links (364-365)
- Info (371, 374)
- Oper/rehash/service (381-383)
- Time/users (391-395)
- All missing error codes (406-415, 422-424, 436-437, 443-446, 463-467, 472, 476-478, 481, 483-485, 491, 501-502)

refs #52

Co-authored-by: clawbot <clawbot@noreply.git.eeqj.de>
Co-authored-by: user <user@Mac.lan guest wan>
Reviewed-on: #71
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

internal/handlers/api.go

@@ -11,7 +11,7 @@ import (
 	"time"
 
 	"git.eeqj.de/sneak/neoirc/internal/db"
-	"git.eeqj.de/sneak/neoirc/internal/irc"
+	"git.eeqj.de/sneak/neoirc/pkg/irc"
 	"github.com/go-chi/chi"
 )
 
@@ -398,12 +398,12 @@ func (hdlr *Handlers) serverName() string {
 func (hdlr *Handlers) enqueueNumeric(
 	ctx context.Context,
 	clientID int64,
-	code int,
+	code irc.IRCMessageType,
 	nick string,
 	params []string,
 	text string,
 ) {
-	command := fmt.Sprintf("%03d", code)
+	command := code.Code()
 
 	body, err := json.Marshal([]string{text})
 	if err != nil {
@@ -969,7 +969,7 @@ func (hdlr *Handlers) respondIRCError(
 	writer http.ResponseWriter,
 	request *http.Request,
 	clientID, sessionID int64,
-	code int,
+	code irc.IRCMessageType,
 	nick string,
 	params []string,
 	text string,