Fix duplicate migration version numbers (000048/000049)

Renumber migrations from merged feature/ai-feature-activation branch
to resolve golang-migrate duplicate version conflict:
- 000048_add_captain_auto_reply_rules → 000050
- 000049_add_article_embedding_vector → 000051

Both migrations were already applied to the database; schema_migrations
version updated from 49 to 51 to reflect the new numbering.

backend/migrations/000051_add_article_embedding_vector.down.sql

@@ -0,0 +1,5 @@
+-- 000051_add_article_embedding_vector.down.sql
+-- Remove the pgvector column from article_embeddings
+
+DROP INDEX IF EXISTS idx_article_embeddings_vector;
+ALTER TABLE article_embeddings DROP COLUMN IF EXISTS vector_embedding;