Make invalid search input explicit in UI and API

templates/players/index.html

@@ -6,6 +6,19 @@
 <section class="panel">
   <h1>Player Search</h1>
   <p class="mt-1 text-sm text-slate-600">Filter players by profile, origin, context, and production metrics.</p>
+  {% if search_has_errors %}
+    <div class="mt-4 rounded-md border border-rose-200 bg-rose-50 p-3 text-sm text-rose-800">
+      <p class="font-medium">Please correct the highlighted filters.</p>
+      {% for field in search_form %}
+        {% for error in field.errors %}
+          <p>{{ field.label }}: {{ error }}</p>
+        {% endfor %}
+      {% endfor %}
+      {% for error in search_form.non_field_errors %}
+        <p>{{ error }}</p>
+      {% endfor %}
+    </div>
+  {% endif %}
 
   <form
     method="get"