fix(search): make FTS sync idempotent to stop duplicate rows on backfill

[tompscanlan]

Makes contrail's D1 full-text-search sync idempotent so repeated backfills stop accumulating duplicate FTS rows.

Root cause

buildFtsStatements only deleted the existing FTS row before inserting when the record was already in existingMap. Backfill runs with skipReplayDetection, which leaves existingMap empty, so every re-applied record looks brand-new and appends another row to the fts_<coll> virtual table (FTS5 has no uniqueness constraint). Re-running a backfill therefore accumulates duplicate FTS rows, and the search JOIN (JOIN fts ON fts.uri = r.uri) then fans each matching record out into one result row per duplicate.

Downstream this surfaced as a hard failure in an appview consumer (atmo): a keyed list ({#each results as r (r.uri)}) throws on the duplicate keys and blanks the page. The records table is unaffected because it uses INSERT ... ON CONFLICT(uri) DO UPDATE.

Fix

Make the delete-then-insert unconditional so FTS sync is idempotent regardless of replay detection, and drop the now-unused existingMap parameter from buildFtsStatements.

Testing

• New failing test added first (TDD): re-applying the same record twice with skipReplayDetection returned 2 rows from queryRecords (RED), then 1 after the fix (GREEN).
• Full @atmo-dev/contrail suite: 438 passed, 3 skipped (Postgres-only); typecheck clean.
• Verified live against a D1 that had accumulated dupes: cleaned 653 duplicate fts_event rows, and the search JOIN for previously-fanning terms now returns rows == distinct_uris.

Note for existing indexes

Deployments that already accumulated duplicates need a one-time cleanup; this fix prevents new ones but does not retro-clean. Run it after deploying the fix, or duplicates re-accumulate.

There is one FTS5 virtual table per searchable collection, named fts_<short> (plus spaces_fts_<short> when spaces mode is used). List the ones that exist:

SELECT name FROM sqlite_master WHERE type='table' AND sql LIKE '%USING fts5%';

Then dedupe each, keeping the lowest rowid per uri. For the default events config that is a single table:

DELETE FROM fts_event WHERE rowid NOT IN (SELECT MIN(rowid) FROM fts_event GROUP BY uri);

(The fts5 shadow tables *_data / *_idx / *_docsize / *_config are maintained automatically; only delete from the virtual table itself.)