fix(#240): add missing V36 index migration and rename needs_expert to V37

V36 (add_index_transcription_blocks_document_id) was applied to the dev
database during a previous local session but never committed to git.
Flyway checksum mismatch prevented the backend from starting.

- V36__add_index_transcription_blocks_document_id.sql: restored from the
  index that already exists in the database (idx_transcription_blocks_document_id)
- V36__add_needs_expert_to_documents.sql → V37__add_needs_expert_to_documents.sql

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

backend/src/main/resources/db/migration/V36__add_index_transcription_blocks_document_id.sql

@@ -0,0 +1 @@
+CREATE INDEX idx_transcription_blocks_document_id ON transcription_blocks(document_id);