feat(groups): remove a member, and keep gigabytes out of the tab

**Removing a member.** The owner can do it from the Members tab, and it
is two halves in the order that fails safe: the node stops serving the
group key first (an operator-signed request, so a paired browser only),
then the hub drops the membership row. The other order would leave
someone able to reach a node that still serves them.

It is a membership, not an account. The user row is never written: their
other groups, their files and their pinned identity survive, because one
group's owner must not be able to erase someone from the hub. It is also
per group — a node hosting two loses them from one — and it does not take
back the key they already unwrapped, which is what rotating the GEK is
for. The confirmation and the panel both say so.

**Downloads and streaming through the disk, in both browsers.** The audit
this started as found two ways to put gigabytes in a tab.

Firefox and Safari have no File System Access API, so every download
there was collected in memory. A service worker fixes it: the page keeps
the writable half of a transferred stream, the worker answers a made-up
URL with the readable half and a Content-Disposition header, and the
browser writes it to disk as it arrives, with real backpressure. The
worker caches nothing and falls through on every request that is not one
of these downloads. A zip announces no Content-Length, since the archive
is larger than the files in it and a length we miss truncates the file.

Video was worse and affected both browsers. The node pushed ffmpeg's
whole output as fast as it was produced while the player consumed a
segment at a time, so the queue held the film — and appending all of it
hit the SourceBuffer's cap, where the handler logged the error and
dropped the segment, leaving a hole in the middle of the film with
nothing to show for it. Streaming is credit-based now, 24 segments of
256 KB in flight, verified against the live node: three credits, three
segments, then silence until more are granted. The player evicts what is
more than a minute behind the playhead and retries a refused segment
rather than dropping it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

1 files changed, 48 insertions, 0 deletions

diff --git a/packages/meshbay-hub/src/meshbay_hub/api/groups.py b/packages/meshbay-hub/src/meshbay_hub/api/groups.py
index 83feeb4..74c6c9e 100644
--- a/packages/meshbay-hub/src/meshbay_hub/api/groups.py
+++ b/packages/meshbay-hub/src/meshbay_hub/api/groups.py
@@ -276,6 +276,54 @@ async def create_group(
     return {"group_id": group.id, "name": group.name}
 
 
+@router.delete("/{group_id}/members/{username}")
+async def remove_group_member(
+    group_id: str,
+    username: str,
+    request: Request,
+    current_user: User = Depends(require_user_scope),
+    db: AsyncSession = Depends(get_db),
+):
+    """
+    Remove someone from a group. The group's owner only.
+
+    This is half of removing a member, and the half the hub can do: without a
+    membership row they cannot reach the node through signaling, and their next
+    token will not name this group. What it does not do is make the node forget
+    them — the node's roster decides who it serves, and only a paired operator
+    can change that (`member_revoke` over MNP, or `meshbay-node member revoke`).
+    The browser does both; a caller using this endpoint alone should know it did
+    one.
+    """
+    group = await db.get(Group, group_id)
+    if not group:
+        raise HTTPException(status_code=404, detail="Group not found")
+    if group.admin_id != current_user.id:
+        raise HTTPException(status_code=403,
+                            detail="Only the group owner can remove members")
+
+    target = (await db.execute(
+        select(User).where(User.username == username))).scalar_one_or_none()
+    if not target:
+        raise HTTPException(status_code=404, detail="User not found")
+    if target.id == group.admin_id:
+        raise HTTPException(
+            status_code=409,
+            detail="The owner cannot be removed from their own group. Hand the "
+                   "group over or delete it.")
+
+    membership = await db.get(GroupMember, (group_id, target.id))
+    if not membership:
+        raise HTTPException(status_code=404, detail="Not a member of this group")
+
+    await db.delete(membership)
+    db.add(IPLog(user_id=current_user.id, event="group_leave",
+                 ip_address=client_ip(request),
+                 detail=f"{username} removed from {group.name}"))
+    await db.commit()
+    return {"status": "removed", "group_id": group_id, "username": username}
+
+
 class GroupUpdateRequest(BaseModel):
     description: str | None = None