fix(database): skip orphaned entries in migration v127

Databases restored from a dump without foreign key enforcement can
contain entries whose feed no longer exists. Migration v127 copied
those rows into entry_tombstones, violating its feed_id foreign key
and aborting the upgrade at schema version 126.

Only backfill tombstones for feeds that still exist: a tombstone for
a deleted feed is never consulted, and the orphaned rows are removed
by the DELETE that follows.

Fixes #4432