feat: funciones Go — core (cron, join_by_key, validate_struct), datascience (pivot, diff_entities), infra (http, cache, cron_ticker)

Nuevas funciones Go con tests en tres dominios:
- core: parse_cron_expr, next_cron_time, join_by_key, validate_struct_fields + tipo CronSchedule
- datascience: pivot (tabla dinámica), diff_entities (comparación de entidades)
- infra: http_get_json, http_post_json, http_download_file, cache_to_sqlite, cron_ticker

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

functions/infra/http_post_json.go

@@ -0,0 +1,63 @@
+package infra
+
+import (
+	"bytes"
+	"encoding/json"
+	"fmt"
+	"io"
+	"net/http"
+	"time"
+)
+
+// HttpPostJSON realiza un POST request con body JSON y parsea la respuesta como JSON.
+// Agrega Content-Type: application/json y Accept: application/json automaticamente.
+// Retorna error si status >= 400 incluyendo status code y los primeros 200 bytes del body.
+func HttpPostJSON(url string, body any, headers map[string]string, timeout time.Duration) (map[string]any, error) {
+	data, err := json.Marshal(body)
+	if err != nil {
+		return nil, fmt.Errorf("http_post_json: marshal body: %w", err)
+	}
+
+	client := &http.Client{Timeout: timeout}
+
+	req, err := http.NewRequest(http.MethodPost, url, bytes.NewReader(data))
+	if err != nil {
+		return nil, fmt.Errorf("http_post_json: build request: %w", err)
+	}
+
+	req.Header.Set("Content-Type", "application/json")
+	req.Header.Set("Accept", "application/json")
+	for k, v := range headers {
+		req.Header.Set(k, v)
+	}
+
+	resp, err := client.Do(req)
+	if err != nil {
+		return nil, fmt.Errorf("http_post_json: %w", err)
+	}
+	defer resp.Body.Close()
+
+	respBody, err := io.ReadAll(resp.Body)
+	if err != nil {
+		return nil, fmt.Errorf("http_post_json: read body: %w", err)
+	}
+
+	if resp.StatusCode >= 400 {
+		preview := respBody
+		if len(preview) > 200 {
+			preview = preview[:200]
+		}
+		shortURL := url
+		if len(shortURL) > 100 {
+			shortURL = shortURL[:100]
+		}
+		return nil, fmt.Errorf("http_post_json: HTTP %d at %q — %s", resp.StatusCode, shortURL, preview)
+	}
+
+	var result map[string]any
+	if err := json.Unmarshal(respBody, &result); err != nil {
+		return nil, fmt.Errorf("http_post_json: parse JSON: %w", err)
+	}
+
+	return result, nil
+}