refactor(sync): no mass backfill — identities are stamped on what is touched

Correcting an over-design of my own. I had contacts given an identity in bulk
the first time synchronisation was switched on, and a full dedupe-key map of the
logbook rebuilt on every pass, both sized for 123 000 contacts.

Neither is needed, because the change log starts EMPTY. Contacts logged before
synchronisation was switched on are never in anyone's log and so are never
exchanged; an identity is stamped only on a contact that is actually logged,
edited or deleted from then on. Seeding a second machine with the existing
history is a one-time copy of the database or an ADIF import — not something
synchronisation should be doing, and not something it can do from a file that
starts empty.

One case survives, and it is why the dedupe key stays. Two machines can already
hold the SAME old contact, the second seeded by that copy or import, with
different row ids and no identity on either. The day one of them edits it, it
stamps an identity and sends a change naming it; the other has never seen that
identity and would insert a duplicate. Matching on the contact itself —
callsign, minute, band, mode, the importer's own key — recognises it.

So that became a targeted lookup through idx_qso_callsign, run only when an
identity is unknown, instead of a whole-table map rebuilt each pass. Rare work,
priced as rare work.

internal/db/migrations/0029_sync_uid.sql

@@ -14,7 +14,11 @@
 -- index a TEXT column without a prefix length, and a migration that tries dies
 -- with error 1170 on every startup thereafter, with no way out from the UI.
 --
--- Empty on every existing row: they are given an identity the first time
--- synchronisation is switched on, in one bulk statement, not row by row.
+-- Empty on every existing row, and it STAYS empty on most of them. The change
+-- log starts empty too, so contacts logged before synchronisation was switched
+-- on are never exchanged: nothing has to be copied across, and an identity is
+-- stamped only on a contact that is actually logged, edited or deleted from
+-- then on. Seeding a second machine with the existing log is a one-time copy of
+-- the database or an ADIF import, not something synchronisation does.
 ALTER TABLE qso ADD COLUMN sync_uid TEXT NOT NULL DEFAULT '';
 CREATE INDEX IF NOT EXISTS idx_qso_sync_uid ON qso (sync_uid);