chore(auth): log JWT header + payload on verify failure

Temporary diagnostic for triaging token-verification 401s. On verify
failure (signature mismatch / expired / kid not in JWKS / wrong aud),
log the underlying error plus a whitelisted decode of the JWT header
(kid, alg, typ) and payload (iss, aud, azp, sub, exp, iat) so the
cause is distinguishable from the log alone.

Only fires on failure — successful requests stay unlogged. The
decodeJWTPart helper whitelists safe metadata fields and never
returns the signature segment.

Remove once the current realm-config drift is settled.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/auth/keycloak.go

@@ -11,8 +11,11 @@ package auth
 import (
 	"context"
 	"crypto/tls"
+	"encoding/base64"
+	"encoding/json"
 	"errors"
 	"fmt"
+	"log"
 	"net/http"
 	"strings"
 	"sync"
@@ -162,6 +165,12 @@ func (vr *Verifier) Middleware() fiber.Handler {
 		raw := strings.TrimPrefix(header, "Bearer ")
 		claims, err := vr.Verify(c.UserContext(), raw)
 		if err != nil {
+			// Temporary diagnostic — log the actual verify error and the
+			// token's header (kid, alg, typ) so we can tell signature vs
+			// expiration vs key-not-found apart. Payload claims (sub, iss,
+			// exp, aud) too so we can correlate with realm config.
+			log.Printf("[auth] verify failed: %v | header=%s payload=%s",
+				err, decodeJWTPart(raw, 0), decodeJWTPart(raw, 1))
 			return c.Status(fiber.StatusUnauthorized).
 				JSON(fiber.Map{"error": fmt.Sprintf("invalid token: %v", err)})
 		}
@@ -170,6 +179,39 @@ func (vr *Verifier) Middleware() fiber.Handler {
 	}
 }
 
+// decodeJWTPart base64-decodes header (idx 0) or payload (idx 1) of a JWT
+// and pretty-prints selected fields. Best-effort — returns the raw segment
+// on parse failure. Never returns the signature segment.
+func decodeJWTPart(token string, idx int) string {
+	parts := strings.Split(token, ".")
+	if idx >= len(parts) || idx > 1 {
+		return "(missing)"
+	}
+	raw, err := base64.RawURLEncoding.DecodeString(parts[idx])
+	if err != nil {
+		return parts[idx][:min(len(parts[idx]), 40)] + "(b64err)"
+	}
+	var m map[string]any
+	if err := json.Unmarshal(raw, &m); err != nil {
+		return string(raw)[:min(len(raw), 80)]
+	}
+	keep := map[string]any{}
+	for _, k := range []string{"kid", "alg", "typ", "iss", "aud", "azp", "sub", "exp", "iat"} {
+		if v, ok := m[k]; ok {
+			keep[k] = v
+		}
+	}
+	b, _ := json.Marshal(keep)
+	return string(b)
+}
+
+func min(a, b int) int {
+	if a < b {
+		return a
+	}
+	return b
+}
+
 func User(c *fiber.Ctx) *UserClaims {
 	if u, ok := c.Locals("user").(*UserClaims); ok {
 		return u