[ychescale9]

Fix a race condition where changing feed filters while a sync was in flight could wipe saved_for_later (and podcast_start_position) on persisted items.

Sync used to read existing items for the currently selected sources, copy over the local state, then rewrite each row with INSERT OR REPLACE. If the selection changed after the request went out, items from sources that were no longer selected weren't in that read, so they were rewritten with saved_for_later = false.

Changes:

• upsertFeedItem now updates only the columns that come from the server, then INSERT OR IGNOREs new rows. Sync no longer touches local-only columns, so there's no pre-read and nothing to carry over. This replaces INSERT OR REPLACE, which deleted and re-inserted rows.
• The feed items sync timestamp is now recorded for the sources the request used, not the selection at write time. Previously a mid-request filter change could mark the new selection as synced with entries fetched for the old one.
• Added insertFeedItem for seeding test data.