feat: Add achievements app with models, admin, and management command for seeding data

backend/server/achievements/models.py

@@ -0,0 +1,33 @@
+from django.db import models
+from django.contrib.auth import get_user_model
+
+User = get_user_model()
+
+VALID_ACHIEVEMENT_TYPES = [
+    "adventure_count",
+    "country_count",
+]
+
+class Achievement(models.Model):
+    """Stores all possible achievements"""
+    name = models.CharField(max_length=255, unique=True)
+    key = models.CharField(max_length=255, unique=True) # Used for frontend lookups, e.g. "achievements.first_adventure"
+    type = models.CharField(max_length=255)  # adventure_count, country_count, etc.
+    description = models.TextField()
+    icon = models.ImageField(upload_to="achievements/", null=True, blank=True)
+    condition = models.JSONField()  # Stores rules like {"type": "adventure_count", "value": 10}
+
+    def __str__(self):
+        return self.name
+
+class UserAchievement(models.Model):
+    """Tracks which achievements a user has earned"""
+    user = models.ForeignKey(User, on_delete=models.CASCADE)
+    achievement = models.ForeignKey(Achievement, on_delete=models.CASCADE)
+    earned_at = models.DateTimeField(auto_now_add=True)
+
+    class Meta:
+        unique_together = ("user", "achievement")  # Prevent duplicates
+
+    def __str__(self):
+        return f"{self.user.username} - {self.achievement.name}"