refactor: unify all C2S commands through POST /messages

All client-to-server commands now go through POST /api/v1/messages
with a 'command' field. The server dispatches by command type:

- PRIVMSG/NOTICE: send message to channel or user
- JOIN: join channel (creates if needed)
- PART: leave channel
- NICK: change nickname
- TOPIC: set channel topic
- PING: keepalive (returns PONG)

Removed separate routes:
- POST /channels/join
- DELETE /channels/{channel}
- POST /register (renamed to POST /session)
- GET /channels/all (moved to GET /channels)

Added DB methods: ChangeNick, SetTopic

internal/server/routes.go

@@ -52,18 +52,16 @@ func (s *Server) SetupRoutes() {
 	// API v1
 	s.router.Route("/api/v1", func(r chi.Router) {
 		r.Get("/server", s.h.HandleServerInfo())
-		r.Post("/register", s.h.HandleRegister())
+		r.Post("/session", s.h.HandleCreateSession())
 
 		// Unified state and message endpoints
 		r.Get("/state", s.h.HandleState())
 		r.Get("/messages", s.h.HandleGetMessages())
-		r.Post("/messages", s.h.HandleSendMessage())
+		r.Post("/messages", s.h.HandleSendCommand())
 		r.Get("/history", s.h.HandleGetHistory())
 
 		// Channels
-		r.Get("/channels/all", s.h.HandleListAllChannels())
-		r.Post("/channels/join", s.h.HandleJoinChannel())
-		r.Delete("/channels/{channel}", s.h.HandlePartChannel())
+		r.Get("/channels", s.h.HandleListAllChannels())
 		r.Get("/channels/{channel}/members", s.h.HandleChannelMembers())
 	})