Admin: allow changing your own quota

The self-protection guard refused every change to the admin's own account.
It now only refuses blocking and deleting yourself; a quota change is fine.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

backend/app/api/admin.py

@@ -3,8 +3,9 @@
 The backend seam for the admin panel: the registration gate — the first account
 on a fresh install becomes the administrator and closes signup behind itself,
 and this is how that gets reopened — the default quota, and managing users
-(search, per-user quota, block, delete). An administrator cannot block or delete
-their own account, so the instance can never lock out its last admin that way.
+(search, per-user quota, block, delete). An administrator can change their own
+quota but cannot block or delete their own account, so the instance can never
+lock out its last admin that way.
 """
 
 from __future__ import annotations
@@ -280,12 +281,15 @@ def _user_out(user: User, used_bytes: int, accounts: list[str]) -> AdminUserOut:
     )
 
 
-async def _target(db: AsyncSession, admin: User, user_id: uuid.UUID) -> User:
-    """The user an admin action is aimed at; never the admin themselves."""
+async def _target(
+    db: AsyncSession, admin: User, user_id: uuid.UUID, *, allow_self: bool = False
+) -> User:
+    """The user an admin action is aimed at. Blocking and deleting are never
+    allowed on the admin themselves; a quota change is (``allow_self``)."""
     user = await db.scalar(select(User).where(User.id == user_id))
     if user is None:
         raise_api_error(404, "not_found", "user not found")
-    if user.id == admin.id:
+    if user.id == admin.id and not allow_self:
         raise_api_error(409, "cannot_modify_self", "you cannot do this to your own account")
     return user
 
@@ -303,7 +307,8 @@ async def patch_user(
     user_id: uuid.UUID, body: AdminUserPatch, admin: CurrentAdmin, db: DbSession
 ) -> AdminUserOut:
     """Change a user's quota and/or block or unblock them."""
-    user = await _target(db, admin, user_id)
+    # Your own quota is fine to change; blocking yourself is not.
+    user = await _target(db, admin, user_id, allow_self=body.blocked is None)
     if body.quota_bytes is not None:
         user.quota_bytes = body.quota_bytes
     if body.blocked is not None: