Persist Bufdir reference number in generated_reports

Add the column via a Supabase migration file: `ALTER TABLE generated_reports ADD COLUMN IF NOT EXISTS bufdir_reference_number TEXT; CREATE INDEX IF NOT EXISTS idx_generated_reports_bufdir_ref ON generated_reports(bufdir_reference_number);`. In GeneratedReportsRepository, add a dedicated `updateBufdirReferenceNumber` method rather than reusing a generic `update` — this makes the intent explicit and avoids accidentally overwriting other fields. Call this method in the BLoC/Cubit submission handler in a try/catch that logs failures but does not rethrow — the reference number write is best-effort after a successful submission. In the export history UI, conditionally render the reference number badge using `if (report.bufdirReferenceNumber != null)`.

Unit tests for GeneratedReportsRepository.updateBufdirReferenceNumber using a Supabase mock or in-memory fake: (1) successful update returns updated record with reference number set, (2) update on non-existent reportId returns error without crashing, (3) null reference number accepted (file-based export path). Integration test against Supabase test project: verify column exists, RLS prevents cross-org write, index present via pg_indexes query. Migration script tested by running against a clean schema and verifying idempotency on second run.