fix(auth): kid-first JWT verification, drop per-attempt debug logging

validateToken logged one line per JWKS key per request (a leftover DEBUG line
plus a per-failed-key line) — with multi-brand certs that produced a log line
per brand cert per request, ~4.4M lines/12h in production, and brute-forced
every key on every call. Now: verify against the key the token's kid header
names (single signature check in the common case), fall back across the
remaining keys only on rotation skew, and emit no per-attempt logs — the
returned error carries the diagnosis.

cmd/kms/auth.go

@@ -149,28 +149,44 @@ func (a *orgJWTAuth) validate(ctx context.Context, raw string) (*orgClaims, erro
 	if err != nil {
 		return nil, fmt.Errorf("jwks: %w", err)
 	}
-	// DEBUG: log JWKS state + token kid for diagnosis
 	tokKid := ""
 	if len(tok.Headers) > 0 {
 		tokKid = tok.Headers[0].KeyID
 	}
-	kidList := make([]string, 0, len(keys.Keys))
-	for _, k := range keys.Keys {
-		kidList = append(kidList, k.KeyID)
-	}
-	log.Printf("kms: validate: tok kid=%s jwks kids=%v", tokKid, kidList)
 	var claims orgClaims
 	verified := false
 	var lastErr error
+	// Try the key the token names first (kid header) — the common case is one
+	// exact match, so verification is a single signature check instead of
+	// brute-forcing every brand cert. A miss on a non-matching key is expected,
+	// not an event: no per-attempt logging (it produced one line per brand cert
+	// per request — millions/day); the returned error carries the diagnosis.
 	for _, k := range keys.Keys {
+		if tokKid != "" && k.KeyID != tokKid {
+			continue
+		}
 		if err := tok.Claims(k.Key, &claims); err == nil {
 			verified = true
 			break
 		} else {
-			log.Printf("kms: validate: kid=%s err=%v", k.KeyID, err)
 			lastErr = err
 		}
 	}
+	if !verified && tokKid != "" {
+		// kid named but didn't verify (rotation skew) — legacy fallback across
+		// the remaining keys before failing.
+		for _, k := range keys.Keys {
+			if k.KeyID == tokKid {
+				continue
+			}
+			if err := tok.Claims(k.Key, &claims); err == nil {
+				verified = true
+				break
+			} else {
+				lastErr = err
+			}
+		}
+	}
 	if !verified {
 		if lastErr == nil {
 			lastErr = errors.New("no key matched")