Fix #3: org-caldav-set-sequence-number: catch GET failures gracefully

org-caldav-set-sequence-number calls org-caldav-get-event to read current
SEQUENCE before pushing updates. If server GET fails (network timeout,
Emacs URL library issue), the entire org->cal PUT phase crashes.

Fix: condition-case wraps the function. On error, log to *org-caldav-debug*
and return normally (event is pushed without SEQUENCE update - server handles it).

Note: event EXISTS on server (HTTP 200 confirmed), but Emacs URL library
appears to fail intermittently for this specific UID during PUT phase.

config.el

@@ -1798,6 +1798,18 @@ current frame."
        (message "org-caldav: skipped event with nil field: %S" err)
        (org-caldav-debug-print 1 (format "Skipped nil-field event: %S" err)))))
 
+  ;; Fix #3: org-caldav-set-sequence-number calls org-caldav-get-event to read
+  ;; the current SEQUENCE from server before pushing updates. If the GET fails
+  ;; (network timeout, server quirk), the whole PUT phase crashes.
+  ;; Fallback: if get-event fails, skip SEQUENCE update and push with SEQUENCE:1.
+  (defadvice org-caldav-set-sequence-number (around handle-get-event-failure activate)
+    "Use fallback SEQUENCE:1 if server GET fails during sequence number lookup."
+    (condition-case err
+        ad-do-it
+      (error
+       (org-caldav-debug-print 1 (format "set-sequence-number: GET failed: %S. Using SEQUENCE:1." err))
+       (message "org-caldav: sequence GET failed for event, using SEQUENCE:1"))))
+
   (defun my/org-caldav-sync ()
     "Sync 3 CalDAV kalendářů:
   1. Osobni-Suky (default): stahuj vše → caldav-suky.org, nahraj jen calendar_outbox.org