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.up.sql

@@ -0,0 +1,25 @@
+-- 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
+-- enabling the <=> (cosine distance) operator for efficient similarity search.
+
+-- Ensure pgvector extension is available (should already be installed).
+CREATE EXTENSION IF NOT EXISTS vector;
+
+-- Add a vector(1536) column to article_embeddings
+ALTER TABLE article_embeddings ADD COLUMN IF NOT EXISTS vector_embedding vector(1536);
+
+-- Create an index for efficient cosine similarity search
+CREATE INDEX IF NOT EXISTS idx_article_embeddings_vector
+  ON article_embeddings USING ivfflat (vector_embedding vector_cosine_ops)
+  WITH (lists = 100);
+
+-- Migrate existing JSONB embeddings to the vector column
+-- This parses the JSON array into a pgvector-compatible format
+UPDATE article_embeddings
+SET vector_embedding = (
+  SELECT array_agg(elem::real)::vector(1536)
+  FROM jsonb_array_elements_text(embedding) AS elem
+)
+WHERE vector_embedding IS NULL AND embedding IS NOT NULL;