Forum, Topic: Make slug unique (which creates an index)

Sentry reported a slow DB query for `Topic.slug`.
At the moment a full table scan is performed.
To fasten things up, the database field is marked as unique.
As a side effect, this will create an index to speed up lookups.

With this commit, on sqlite the explain changes

from
```
SCAN forum_topic
```

to
```
SEARCH forum_topic USING INDEX sqlite_autoindex_forum_topic_1 (slug=?)
```

Even if it was not enforced in the database, the slug is unique across
all topics on forum.ubuntuusers.de.

Author: chris34

ChangeLog.rst

@@ -45,6 +45,7 @@ Deployment notes
 * Fix deprecation warnings related to UTC methods
 * Remove private messages after specific duration. This will not affect messages in the 'archive' folder and team members.
 * Event: Slug is changed for unpublished events
+* Forum, Topic: Make slug unique which creates an index and fasten look ups
 
 🗑 Deprecations
 --------------

inyoka/forum/migrations/0018_add_index__topic_slug.py

@@ -0,0 +1,18 @@
+# Generated by Django 4.2.18 on 2025-02-14 19:31
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("forum", "0017_alter_poll_start_time_alter_post_pub_date_and_more"),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name="topic",
+            name="slug",
+            field=models.CharField(blank=True, max_length=50, unique=True),
+        ),
+    ]

inyoka/forum/models.py

@@ -569,7 +569,7 @@ class Topic(models.Model):
     objects = TopicManager()
 
     title = models.CharField(max_length=TITLE_MAX_LENGTH, blank=True)
-    slug = models.CharField(max_length=50, blank=True)
+    slug = models.CharField(max_length=50, blank=True, unique=True)
     view_count = models.IntegerField(default=0)
     sticky = models.BooleanField(default=False, db_index=True)
     solved = models.BooleanField(default=False)