feat(store): migration 019 -- uploaded_transcripts.reconciliation

JSONB, NULL for a single engine so the page cannot claim a second opinion. On the
already FORCE'd, RLS-scoped table, so the exact-set isolation test is unaffected.
Up and down are one commit: golang-migrate pairs them.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

internal/adapters/store/migrations/019_uploaded_transcripts_reconciliation.down.sql

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+ALTER TABLE uploaded_transcripts DROP COLUMN reconciliation;

internal/adapters/store/migrations/019_uploaded_transcripts_reconciliation.up.sql

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+-- Migration 019: how a merged transcript came about (ADR-034).
+--
+-- When two engines transcribed the same audio and a local model chose between their
+-- disagreements, this records the status, the exact counts, and a bounded list of the
+-- contested spots (changed or unsure, each with a position and both readings) so the
+-- page can say what was done and the user can check the uncertain spots against the
+-- audio. NULL means a single engine: no second opinion, and the page must not claim one.
+--
+-- It lives on the already-FORCE'd, RLS-scoped uploaded_transcripts, so the exact-set
+-- isolation test is unaffected. The contested spots hold fragments of the private
+-- transcript, which is why they are here and nowhere else.
+ALTER TABLE uploaded_transcripts ADD COLUMN reconciliation JSONB;