🗃️(backend) add an index on user names and emails

This adds an unaccented gin index on users full_name an email fields.
Using a manual migration because we need to make a wrapper around
unaccent to make it IMMUTABLE (cf.
https://stackoverflow.com/questions/9063402/ )

Author: Ash-Crow

src/backend/core/api/viewsets.py

@@ -210,8 +210,8 @@ def get_queryset(self):
 
         return (
             queryset.annotate(
-                sim_email=TrigramSimilarity(Unaccent("email"), query),
-                sim_name=TrigramSimilarity(Unaccent("full_name"), query),
+                sim_email=TrigramSimilarity("email", query),
+                sim_name=TrigramSimilarity("full_name", query),
             )
             .annotate(similarity=Greatest("sim_email", "sim_name"))
             .filter(similarity__gt=0.2)

src/backend/core/migrations/0026_auto_20251120_0956.py

@@ -0,0 +1,38 @@
+# Generated by Django 5.2.8 on 2025-11-20 09:56
+
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("core", "0025_alter_user_short_name"),
+    ]
+
+    operations = [
+        migrations.RunSQL(
+            sql="""
+                CREATE OR REPLACE FUNCTION public.immutable_unaccent(regdictionary, text)
+                RETURNS text
+                LANGUAGE c IMMUTABLE PARALLEL SAFE STRICT AS
+                '$libdir/unaccent', 'unaccent_dict';
+
+                CREATE OR REPLACE FUNCTION public.f_unaccent(text)
+                RETURNS text
+                LANGUAGE sql IMMUTABLE PARALLEL SAFE STRICT
+                RETURN public.immutable_unaccent(regdictionary 'public.unaccent', $1);
+
+                CREATE INDEX IF NOT EXISTS user_email_unaccent_trgm_idx
+                ON impress_user
+                USING gin (f_unaccent(email) gin_trgm_ops);
+
+                CREATE INDEX IF NOT EXISTS user_full_name_unaccent_trgm_idx
+                ON impress_user
+                USING gin (f_unaccent(full_name) gin_trgm_ops);
+            """,
+            reverse_sql="""
+                DROP INDEX IF EXISTS user_email_unaccent_trgm_idx;
+                DROP INDEX IF EXISTS user_full_name_unaccent_trgm_idx;
+            """,
+        ),
+    ]