refactor: replace Bearer token auth with HttpOnly cookies

- Remove POST /api/v1/register endpoint entirely
- Session creation (POST /api/v1/session) now sets neoirc_auth HttpOnly
  cookie instead of returning token in JSON body
- Login (POST /api/v1/login) now sets neoirc_auth HttpOnly cookie
  instead of returning token in JSON body
- Add PASS IRC command for setting session password (enables multi-client
  login via POST /api/v1/login)
- All per-request auth reads from neoirc_auth cookie instead of
  Authorization: Bearer header
- Cookie properties: HttpOnly, SameSite=Strict, Secure when behind TLS
- Logout and QUIT clear the auth cookie
- Update CORS to AllowCredentials:true with origin reflection
- Remove Authorization from CORS AllowedHeaders
- Update CLI client to use cookie jar (net/http/cookiejar)
- Remove Token field from SessionResponse
- Add SetPassword to DB layer, remove RegisterUser
- Comprehensive test updates for cookie-based auth
- Add tests: TestPassCommand, TestPassCommandShortPassword,
  TestPassCommandEmpty, TestSessionCookie
- Update README extensively: auth model, API reference, curl examples,
  security model, design principles, roadmap

closes #83

internal/handlers/auth.go

@@ -5,120 +5,11 @@ import (
 	"net/http"
 	"strings"
 
-	"git.eeqj.de/sneak/neoirc/internal/db"
+	"git.eeqj.de/sneak/neoirc/pkg/irc"
 )
 
 const minPasswordLength = 8
 
-// HandleRegister creates a new user with a password.
-func (hdlr *Handlers) HandleRegister() http.HandlerFunc {
-	return func(
-		writer http.ResponseWriter,
-		request *http.Request,
-	) {
-		request.Body = http.MaxBytesReader(
-			writer, request.Body, hdlr.maxBodySize(),
-		)
-
-		hdlr.handleRegister(writer, request)
-	}
-}
-
-func (hdlr *Handlers) handleRegister(
-	writer http.ResponseWriter,
-	request *http.Request,
-) {
-	type registerRequest struct {
-		Nick     string `json:"nick"`
-		Password string `json:"password"`
-	}
-
-	var payload registerRequest
-
-	err := json.NewDecoder(request.Body).Decode(&payload)
-	if err != nil {
-		hdlr.respondError(
-			writer, request,
-			"invalid request body",
-			http.StatusBadRequest,
-		)
-
-		return
-	}
-
-	payload.Nick = strings.TrimSpace(payload.Nick)
-
-	if !validNickRe.MatchString(payload.Nick) {
-		hdlr.respondError(
-			writer, request,
-			"invalid nick format",
-			http.StatusBadRequest,
-		)
-
-		return
-	}
-
-	if len(payload.Password) < minPasswordLength {
-		hdlr.respondError(
-			writer, request,
-			"password must be at least 8 characters",
-			http.StatusBadRequest,
-		)
-
-		return
-	}
-
-	sessionID, clientID, token, err :=
-		hdlr.params.Database.RegisterUser(
-			request.Context(),
-			payload.Nick,
-			payload.Password,
-		)
-	if err != nil {
-		hdlr.handleRegisterError(
-			writer, request, err,
-		)
-
-		return
-	}
-
-	hdlr.stats.IncrSessions()
-	hdlr.stats.IncrConnections()
-
-	hdlr.deliverMOTD(request, clientID, sessionID, payload.Nick)
-
-	hdlr.respondJSON(writer, request, map[string]any{
-		"id":    sessionID,
-		"nick":  payload.Nick,
-		"token": token,
-	}, http.StatusCreated)
-}
-
-func (hdlr *Handlers) handleRegisterError(
-	writer http.ResponseWriter,
-	request *http.Request,
-	err error,
-) {
-	if db.IsUniqueConstraintError(err) {
-		hdlr.respondError(
-			writer, request,
-			"nick already taken",
-			http.StatusConflict,
-		)
-
-		return
-	}
-
-	hdlr.log.Error(
-		"register user failed", "error", err,
-	)
-	hdlr.respondError(
-		writer, request,
-		"internal error",
-		http.StatusInternalServerError,
-	)
-}
-
 // HandleLogin authenticates a user with nick and password.
 func (hdlr *Handlers) HandleLogin() http.HandlerFunc {
 	return func(
@@ -195,9 +86,66 @@ func (hdlr *Handlers) handleLogin(
 		request, clientID, sessionID, payload.Nick,
 	)
 
+	hdlr.setAuthCookie(writer, request, token)
+
 	hdlr.respondJSON(writer, request, map[string]any{
-		"id":    sessionID,
-		"nick":  payload.Nick,
-		"token": token,
+		"id":   sessionID,
+		"nick": payload.Nick,
 	}, http.StatusOK)
 }
+
+// handlePass handles the IRC PASS command to set a
+// password on the authenticated session, enabling
+// multi-client login via POST /api/v1/login.
+func (hdlr *Handlers) handlePass(
+	writer http.ResponseWriter,
+	request *http.Request,
+	sessionID, clientID int64,
+	nick string,
+	bodyLines func() []string,
+) {
+	lines := bodyLines()
+	if len(lines) == 0 || lines[0] == "" {
+		hdlr.respondIRCError(
+			writer, request, clientID, sessionID,
+			irc.ErrNeedMoreParams, nick,
+			[]string{irc.CmdPass},
+			"Not enough parameters",
+		)
+
+		return
+	}
+
+	password := lines[0]
+
+	if len(password) < minPasswordLength {
+		hdlr.respondIRCError(
+			writer, request, clientID, sessionID,
+			irc.ErrNeedMoreParams, nick,
+			[]string{irc.CmdPass},
+			"Password must be at least 8 characters",
+		)
+
+		return
+	}
+
+	err := hdlr.params.Database.SetPassword(
+		request.Context(), sessionID, password,
+	)
+	if err != nil {
+		hdlr.log.Error(
+			"set password failed", "error", err,
+		)
+		hdlr.respondError(
+			writer, request,
+			"internal error",
+			http.StatusInternalServerError,
+		)
+
+		return
+	}
+
+	hdlr.respondJSON(writer, request,
+		map[string]string{"status": "ok"},
+		http.StatusOK)
+}