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.xcodeproj/project.pbxproj

@@ -511,7 +511,7 @@
 					"$(inherited)",
 					"@executable_path/Frameworks",
 				);
-				MARKETING_VERSION = 2.8;
+				MARKETING_VERSION = 2.9;
 				PRODUCT_BUNDLE_IDENTIFIER = com.scarriffleservices.calendarr.ios;
 				PRODUCT_NAME = "Calendarr iOS";
 				STRING_CATALOG_GENERATE_SYMBOLS = YES;
@@ -555,7 +555,7 @@
 					"$(inherited)",
 					"@executable_path/Frameworks",
 				);
-				MARKETING_VERSION = 2.8;
+				MARKETING_VERSION = 2.9;
 				PRODUCT_BUNDLE_IDENTIFIER = com.scarriffleservices.calendarr.ios;
 				PRODUCT_NAME = "Calendarr iOS";
 				STRING_CATALOG_GENERATE_SYMBOLS = YES;