frontend: date UTC convertite in ora italiana alla presentazione (#42)

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

src/frontend/flask_app/app.py

@@ -2,7 +2,8 @@
 import json
 import os
 import re
-from datetime import date
+from datetime import date, datetime, timezone
+from zoneinfo import ZoneInfo
 from urllib.parse import urlparse
 
 from flask import Flask, flash, g, jsonify, redirect, url_for, session, request
@@ -154,6 +155,26 @@ def create_app() -> Flask:
         bolded = _BOLD_RX.sub(r"<strong>\1</strong>", escaped)
         return Markup(bolded.replace("\n", "<br>"))
 
+    @app.template_filter("local_dt")
+    def local_dt_filter(value, fmt="%d/%m/%Y %H:%M"):
+        """Show a UTC datetime (ISO string or datetime) in DISPLAY_TIMEZONE.
+
+        A value without timezone is taken as UTC. None or empty gives "-".
+        """
+        if value is None or value == "":
+            return "-"
+        if isinstance(value, str):
+            try:
+                value = datetime.fromisoformat(value.strip().replace("Z", "+00:00"))
+            except ValueError:
+                return value
+        if not isinstance(value, datetime):
+            return str(value)
+        if value.tzinfo is None:
+            value = value.replace(tzinfo=timezone.utc)
+        tz = ZoneInfo(app.config.get("DISPLAY_TIMEZONE") or "Europe/Rome")
+        return value.astimezone(tz).strftime(fmt)
+
     @app.template_filter("tojson_attr")
     def tojson_attr_filter(value):
         """JSON encode safe for HTML attributes (x-data, etc.).