Show the date a link was added

The created_at timestamp was already stored but never displayed. Add
a "datum" Jinja filter that converts the stored UTC value to local
time and render it on both the link card and the edit form.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

app/main.py

@@ -1,4 +1,5 @@
 import json
+from datetime import datetime, timezone
 from pathlib import Path
 
 from fastapi import Depends, FastAPI, Form, Request
@@ -26,6 +27,18 @@ templates.env.globals.update(
 )
 
 
+def format_datum(value: datetime | None) -> str:
+    """Formatiere einen UTC-Zeitstempel als lokale Datums-/Zeitangabe."""
+    if not value:
+        return ""
+    if value.tzinfo is None:
+        value = value.replace(tzinfo=timezone.utc)
+    return value.astimezone().strftime("%d.%m.%Y, %H:%M")
+
+
+templates.env.filters["datum"] = format_datum
+
+
 # ---------------------------------------------------------------------------
 # Hilfsfunktionen
 # ---------------------------------------------------------------------------