Phase 3.1: Help Center semantic search with pgvector

- ArticleEmbeddingRepo (new): Upsert, GetByArticleID, DeleteByArticleID,
  SearchByEmbedding using pgvector cosine distance (vector_embedding column)
- ArticleEmbedding model: add VectorEmbedding pgvector.Vector field
  alongside existing JSONB Embedding (backward compatible)
- ArticleService: add SemanticSearch() — generates query embedding via LLM,
  searches articles by cosine similarity; add GenerateEmbedding() — creates
  and stores article embedding from title+description+content
- ArticleHandler: add SemanticSearch endpoint
  GET /portals/:portal_id/articles/semantic_search?query=...
- bootstrap.go: inject articleEmbeddingRepo + llmProvider into ArticleService
- router.go: register /articles/semantic_search route
- migration 000049: add vector(1536) column to article_embeddings table,
  create ivfflat index, migrate existing JSONB data to vector format

Verified: go build + go vet + go test all pass

backend/internal/handler/api/v1/article_handler.go

@@ -405,6 +405,45 @@ func (h *ArticleHandler) Search(c *gin.Context) {
 	c.JSON(http.StatusOK, gin.H{"payload": articlePayloads(articles), "meta": articleListMetaPayload(meta, p.Page)})
 }
 
+// SemanticSearch performs AI-powered semantic search on help center articles.
+// Uses LLM embeddings + pgvector cosine similarity to find articles by meaning,
+// not just keyword matching.
+//
+// GET /portals/:portal_id/articles/semantic_search?query=how+to+reset+password
+func (h *ArticleHandler) SemanticSearch(c *gin.Context) {
+	accountID := getAccountID(c)
+	if accountID == 0 {
+		response.AbortWithStatusError(c, http.StatusBadRequest, response.ErrBadRequest, "invalid account_id")
+		return
+	}
+	portal, ok := h.resolvePortal(c, accountID)
+	if !ok {
+		return
+	}
+
+	query := strings.TrimSpace(c.Query("query"))
+	if query == "" {
+		response.AbortWithStatusError(c, http.StatusBadRequest, response.ErrBadRequest, "query parameter is required")
+		return
+	}
+
+	limit := 10
+	if l := c.Query("limit"); l != "" {
+		if parsed, err := strconv.Atoi(l); err == nil && parsed > 0 && parsed <= 50 {
+			limit = parsed
+		}
+	}
+
+	articles, err := h.svc.SemanticSearch(c.Request.Context(), portal.ID, query, limit)
+	if err != nil {
+		applogger.L().Errorf("SemanticSearch articles: %v", err)
+		response.AbortWithStatusError(c, http.StatusInternalServerError, response.ErrInternal, "failed to perform semantic search")
+		return
+	}
+
+	c.JSON(http.StatusOK, gin.H{"payload": articlePayloads(articles)})
+}
+
 // StatusCounts returns article counts grouped by status.
 // GET /portals/:portal_id/articles/status_counts
 func (h *ArticleHandler) StatusCounts(c *gin.Context) {