feat(cluster): Fix "joining" status + cross-node auth federation

1. preRegisterJoiner now runs SYNCHRONOUSLY before IssueCert responds,
   so nftables @peer_ipv4 is updated before the joiner calls autoRegister.
   Previously it was a goroutine → race → autoRegister failed → "joining"
   forever.

2. Stable node ID for pre-registered placeholder (prenode-{fqdn}) instead
   of time-based ID — re-joins are now idempotent.

3. AgentRegisterPeer sets status="online" immediately (peer proved it is
   online by connecting via mTLS) and deletes the prenode-{fqdn} placeholder.

4. autoRegister retries 3× with 2s delay in case of transient nftables lag.

5. Auth federation: cluster nodes forward failed logins to the primary via
   mTLS /agent/auth/check so users can log in on any node with primary
   credentials (no PG replication needed).
   - SystemHandler.AgentAuthCheck: new endpoint on :8443
   - AuthHandler.checkWithPrimary: mTLS call to primary when local auth fails
   - AuthHandler.WithClusterTLS: inject cluster TLS store
   - startAgentListener now uses the wired systemHdl with Users repo

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/handlers/auth.go

@@ -1,13 +1,19 @@
 package handlers
 
 import (
+	"bytes"
+	"context"
+	"encoding/json"
 	"errors"
+	"io"
+	"log/slog"
 	"net/http"
 	"strings"
 	"time"
 
 	"github.com/gin-gonic/gin"
 
+	"git.netcell-it.de/projekte/edgeguard-native/internal/cluster/clustertls"
 	"git.netcell-it.de/projekte/edgeguard-native/internal/handlers/response"
 	"git.netcell-it.de/projekte/edgeguard-native/internal/services/audit"
 	"git.netcell-it.de/projekte/edgeguard-native/internal/services/session"
@@ -21,11 +27,12 @@ import (
 // the account is auto-migrated into the DB (Upsert) so it shows up in
 // user management from that point on.
 type AuthHandler struct {
-	Setup  *setup.Store
-	Signer *session.Signer
-	Audit  *audit.Repo
-	NodeID string
-	Users  *usersvc.Repo // optional — nil on first boot before DB is ready
+	Setup      *setup.Store
+	Signer     *session.Signer
+	Audit      *audit.Repo
+	NodeID     string
+	Users      *usersvc.Repo     // optional — nil on first boot before DB is ready
+	ClusterTLS *clustertls.Store // optional — enables auth federation on cluster nodes
 }
 
 func NewAuthHandler(s *setup.Store, sig *session.Signer) *AuthHandler {
@@ -46,6 +53,13 @@ func (h *AuthHandler) WithUsers(u *usersvc.Repo) *AuthHandler {
 	return h
 }
 
+// WithClusterTLS enables auth federation: when local auth fails on a
+// cluster node, Login tries the primary via mTLS /agent/auth/check.
+func (h *AuthHandler) WithClusterTLS(store *clustertls.Store) *AuthHandler {
+	h.ClusterTLS = store
+	return h
+}
+
 // Register mounts /auth/login + /logout (public) and /auth/me
 // (gated by requireAuth, passed in as a per-route middleware).
 func (h *AuthHandler) Register(rg *gin.RouterGroup, requireAuth gin.HandlerFunc) {
@@ -115,24 +129,38 @@ func (h *AuthHandler) Login(c *gin.Context) {
 	}
 
 	// 2. Fallback: setup-store admin (backwards compat for pre-DB installs).
-	if actor == "" {
-		if !strings.EqualFold(st.AdminEmail, email) || !st.VerifyAdminPassword(req.Password) {
-			if h.Audit != nil {
-				_ = h.Audit.Log(c.Request.Context(), email, "auth.login.failed",
-					email, gin.H{"reason": "invalid_credentials", "remote": remote}, h.NodeID)
+	if actor == "" && st.AdminEmail != "" {
+		if strings.EqualFold(st.AdminEmail, email) && st.VerifyAdminPassword(req.Password) {
+			actor = st.AdminEmail
+			role = "admin"
+			// Auto-migrate: insert the setup-store admin into the DB so it
+			// shows up in user management from this point on.
+			if h.Users != nil {
+				_, _ = h.Users.Upsert(c.Request.Context(), st.AdminEmail, req.Password, "admin", true)
 			}
-			response.Unauthorized(c, errors.New("invalid_credentials"))
-			return
 		}
-		actor = st.AdminEmail
-		role = "admin"
-		// Auto-migrate: insert the setup-store admin into the DB so it
-		// shows up in user management from this point on.
-		if h.Users != nil {
-			_, _ = h.Users.Upsert(c.Request.Context(), st.AdminEmail, req.Password, "admin", true)
+	}
+
+	// 3. Auth federation: cluster nodes forward failed auth to the primary
+	// via mTLS so users can log in with their primary credentials on any node.
+	if actor == "" && st.IsClusterNode && st.PrimaryFQDN != "" && h.ClusterTLS != nil {
+		if a, r, err := h.checkWithPrimary(c.Request.Context(), st.PrimaryFQDN, email, req.Password); err == nil {
+			actor = a
+			role = r
+		} else {
+			slog.Debug("auth: primary auth check failed", "primary", st.PrimaryFQDN, "error", err)
 		}
 	}
 
+	if actor == "" {
+		if h.Audit != nil {
+			_ = h.Audit.Log(c.Request.Context(), email, "auth.login.failed",
+				email, gin.H{"reason": "invalid_credentials", "remote": remote}, h.NodeID)
+		}
+		response.Unauthorized(c, errors.New("invalid_credentials"))
+		return
+	}
+
 	raw, tok, err := h.Signer.IssueWithRole(actor, role)
 	if err != nil {
 		response.Internal(c, err)
@@ -278,6 +306,48 @@ func (h *AuthHandler) ChangePassword(c *gin.Context) {
 	response.OK(c, gin.H{"ok": true})
 }
 
+// checkWithPrimary verifies credentials against the primary node via mTLS.
+// Returns actor+role on success, error on failure.
+func (h *AuthHandler) checkWithPrimary(ctx context.Context, primaryFQDN, email, password string) (string, string, error) {
+	clientTLS, err := h.ClusterTLS.ClientTLSConfig()
+	if err != nil {
+		return "", "", err
+	}
+	tr := &http.Transport{TLSClientConfig: clientTLS, TLSHandshakeTimeout: 5 * time.Second}
+	client := &http.Client{Transport: tr, Timeout: 8 * time.Second}
+
+	body, _ := json.Marshal(map[string]string{"email": email, "password": password})
+	reqURL := "https://" + primaryFQDN + ":8443/agent/auth/check"
+	req, err := http.NewRequestWithContext(ctx, http.MethodPost, reqURL, bytes.NewReader(body))
+	if err != nil {
+		return "", "", err
+	}
+	req.Header.Set("Content-Type", "application/json")
+
+	resp, err := client.Do(req)
+	if err != nil {
+		return "", "", err
+	}
+	defer resp.Body.Close()
+	raw, _ := io.ReadAll(io.LimitReader(resp.Body, 64*1024))
+	if resp.StatusCode != http.StatusOK {
+		return "", "", errors.New("primary: " + strings.TrimSpace(string(raw)))
+	}
+	var env struct {
+		Data struct {
+			Actor string `json:"actor"`
+			Role  string `json:"role"`
+		} `json:"data"`
+	}
+	if err := json.Unmarshal(raw, &env); err != nil {
+		return "", "", err
+	}
+	if env.Data.Actor == "" {
+		return "", "", errors.New("primary returned empty actor")
+	}
+	return env.Data.Actor, env.Data.Role, nil
+}
+
 func setSessionCookie(c *gin.Context, raw string, expUnix int64) {
 	maxAge := int(time.Until(time.Unix(expUnix, 0)).Seconds())
 	if maxAge < 0 {