docs: add Lunr.js search with fuzzy matching and highlighting

Phase 4 part 1: Add search infrastructure
- Add SearchDoc struct and GenerateSearchIndex to docs handler
- Add stripHTML helper for plain text extraction
- Add ServeSearchIndex endpoint to http handler
- Add /docs/search-index.json route in main.go
- Search index includes all documentation files with ID, title, content, URL

internal/docs/http_handler.go

@@ -332,3 +332,14 @@ func (h *HTTPHandler) TryEndpoint(c echo.Context) error {
 		"message": "API explorer not yet implemented",
 	})
 }
+
+// ServeSearchIndex serves the Lunr.js search index
+func (h *HTTPHandler) ServeSearchIndex(c echo.Context) error {
+	index, err := h.docs.GenerateSearchIndex()
+	if err != nil {
+		return c.JSON(http.StatusInternalServerError, map[string]string{
+			"error": "failed to generate search index",
+		})
+	}
+	return c.JSON(http.StatusOK, index)
+}