feat: store finding as structured title/fix/sources, not flat message

Migration 0002 replaces finding.message with title/fix/sources (a
Postgres text[]). A finding needs to render into the dossier the way it
looked at the moment it was raised — referencing the current rules/*.yaml
by rule_id+version isn't safe once that file is edited for a later
version, since old wording isn't kept around as a separate live file.
Added as a new migration rather than editing 0001, since that's already
applied on the test server.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

internal/store/migrations/0002_finding_structured_content.down.sql

@@ -0,0 +1,5 @@
+ALTER TABLE finding DROP COLUMN title;
+ALTER TABLE finding DROP COLUMN fix;
+ALTER TABLE finding DROP COLUMN sources;
+ALTER TABLE finding ADD COLUMN message TEXT NOT NULL DEFAULT '';
+ALTER TABLE finding ALTER COLUMN message DROP DEFAULT;