v0.7.1: in-app password change, custom labels, always-on folders

- Settings -> Account: change your own password (current + new), session
  preserved via update_session_auth_hash.
- Custom labels: per-user palette in Settings -> Labels; apply/remove from a
  message's ... menu (menu stays open, checkmark = applied). Labels are unified
  with tags; chip colours sync to the global TagColor. New Label model +
  migration 0017; POST /a/{id}/label (session, owner-scoped, palette-validated).
- Sidebar Keep/Deleted folders are now always shown; count badge only when
  non-empty. Updates the stale 'hide when empty' note in CLAUDE.md.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

artifacts/models.py

@@ -96,6 +96,23 @@ class TagColor(models.Model):
         return f"{self.name} ({self.color})"
 
 
+class Label(models.Model):
+    """A user's reusable label in their personal palette. Applying one from the
+    feed UI just adds its `name` to an artifact's `tags`; the chip colour lives
+    in the global TagColor table (kept in sync when the palette entry is saved)."""
+
+    user = models.ForeignKey(settings.AUTH_USER_MODEL, on_delete=models.CASCADE, related_name="labels")
+    name = models.CharField(max_length=40)
+    color = models.CharField(max_length=20)  # #RRGGBB
+
+    class Meta:
+        unique_together = ("user", "name")
+        ordering = ["name"]
+
+    def __str__(self):
+        return f"{self.name} ({self.color})"
+
+
 class UserProfile(models.Model):
     DEFAULT_QUOTA_BYTES = 1024 * 1024 * 1024  # 1 GB