fix(ics): export a series' deleted occurrences (#225)

Import learned to read EXDATE, but the export side never wrote it: an in-app
backup and restore brought back every occurrence the user had deleted. EXDATE
lives on the master row, so the export query's ORIGINAL_ID IS NULL filter never
hid it — it just wasn't in the projection.

An all-day series' exclusions are written VALUE=DATE, since RFC 5545 ties
EXDATE's value type to DTSTART's and a bare day code without it reads as a
malformed DATE-TIME. Sync adapters disagree on whether an all-day exclusion is
yyyyMMdd or a padded midnight stamp, so the time part is dropped on the way out.

CHANGELOG.md

@@ -42,6 +42,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   imported without complaint and then appeared nowhere. Any all-day event that
   ends where it starts, or carries no end at all, is now a one-day event
   ([#225]).
+- **A backup no longer brings back occurrences you had deleted.** Remove a single
+  occurrence of a repeating event, back up, restore — and it was there again,
+  because the removals never made it into the file. They travel with the series
+  now, so a restored calendar looks the way you left it ([#225]).
 
 ### Added
 - **More of an imported `.ics` survives the trip**: tasks come across as events