fix: preserve cached events when a calendar has a per-source sync error

Root cause: mergeIntoCache() unconditionally evicted every event in the
fetched date range, even for calendars whose sync failed. Those calendars'
events were removed and never restored, so the calendar appeared to vanish.

Fix:
- SyncError now decodes calendarId from the server's "calendar_id" field
  (added server-side in e0ea16f but not yet read by iOS).
- mergeIntoCache() gains a keepKeysInRange parameter: events from failed
  calendars are retained (not evicted) even within the fetch window.
- loadEvents() and prefetchBackground() compute the failed keys from the
  syncErrors list and pass them to mergeIntoCache.

Result: when CalDAV / Google / HA sync fails for a specific calendar, the
user sees stale-but-correct events alongside the existing error banner,
instead of a completely empty calendar with no explanation.

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

Calendarr iOS/Models/CalEvent.swift

@@ -25,16 +25,18 @@ struct EventPerson: Hashable {
 /// succeeded, just not every source within it.
 struct SyncError: Hashable {
     let source: String
+    let calendarId: String?   // nil on older server responses without calendar_id
     let name: String
     let message: String
 
     static func from(json: [String: Any]) -> SyncError? {
         guard
-            let source = json["source"] as? String,
-            let name = json["name"] as? String,
+            let source  = json["source"]  as? String,
+            let name    = json["name"]    as? String,
             let message = json["message"] as? String
         else { return nil }
-        return SyncError(source: source, name: name, message: message)
+        let calendarId = json["calendar_id"].map { "\($0)" }
+        return SyncError(source: source, calendarId: calendarId, name: name, message: message)
     }
 }