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/000050_add_captain_auto_reply_rules.down.sql

@@ -1,4 +1,4 @@
--- 000048_add_captain_auto_reply_rules.down.sql
+-- 000050_add_captain_auto_reply_rules.down.sql
 -- Drop Captain AI auto-reply rules table
 
 DROP TABLE IF EXISTS captain_auto_reply_rules;

backend/migrations/000050_add_captain_auto_reply_rules.up.sql

@@ -1,4 +1,4 @@
--- 000048_add_captain_auto_reply_rules.up.sql
+-- 000050_add_captain_auto_reply_rules.up.sql
 -- Captain AI — Auto-Reply Rules table
 -- Reference: M12 PRD §Captain AI — Auto-Reply Rules
 -- Stores rules for automated replies when incoming messages match conditions.

backend/migrations/000051_add_article_embedding_vector.down.sql

@@ -1,4 +1,4 @@
--- 000049_add_article_embedding_vector.down.sql
+-- 000051_add_article_embedding_vector.down.sql
 -- Remove the pgvector column from article_embeddings
 
 DROP INDEX IF EXISTS idx_article_embeddings_vector;

backend/migrations/000051_add_article_embedding_vector.up.sql

@@ -1,4 +1,4 @@
--- 000049_add_article_embedding_vector.up.sql
+-- 000051_add_article_embedding_vector.up.sql
 -- Add pgvector vector column to article_embeddings for semantic search.
 -- The original embedding JSONB column is kept for backward compatibility;
 -- the new vector_column stores the same data as a native pgvector type