fix(db): add missing schema_version insert to migration 019

Migration 019 created performance indexes but never recorded itself
in the schema_version table. Without this row the migration runner
considers the schema outdated and would attempt to re-apply. Adds
the standard INSERT INTO schema_version for version 19.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

migrations/019_list_performance.sql

@@ -11,3 +11,6 @@ CREATE INDEX IF NOT EXISTS idx_mrs_updated_at_desc
 -- MRs already have idx_discussions_mr_resolved (migration 006).
 CREATE INDEX IF NOT EXISTS idx_discussions_issue_resolved
     ON discussions(issue_id, resolvable, resolved);
+
+INSERT INTO schema_version (version, applied_at, description)
+VALUES (19, strftime('%s', 'now') * 1000, 'List performance indexes');