fix: stable event hashing, HEAD fallback, caldav str/bytes compat

diff.py

@@ -5,6 +5,19 @@ from icalendar import Calendar
 logger = logging.getLogger(__name__)
 
 
+def _stable_event_key(component) -> str:
+    event_bytes = component.to_ical()
+    text = event_bytes.decode("utf-8", errors="replace")
+    stable_lines = []
+    for line in text.split("\r\n"):
+        key = line.split(":")[0].upper() if ":" in line else ""
+        if key in ("DTSTAMP", "LAST-MODIFIED"):
+            continue
+        stable_lines.append(line)
+    stable_text = "\r\n".join(stable_lines)
+    return hashlib.sha256(stable_text.encode("utf-8")).hexdigest()
+
+
 def parse_ics_events(ics_text: str) -> dict[str, str]:
     cal = Calendar.from_ical(ics_text.encode() if isinstance(ics_text, str) else ics_text)
     result = {}
@@ -17,9 +30,7 @@ def parse_ics_events(ics_text: str) -> dict[str, str]:
             continue
         uid_str = str(uid)
         try:
-            event_bytes = component.to_ical()
-            file_hash = hashlib.sha256(event_bytes).hexdigest()
-            result[uid_str] = file_hash
+            result[uid_str] = _stable_event_key(component)
         except Exception as e:
             logger.warning("Failed to process event %s: %s", uid_str, e)
     return result