merge: Phase 11.5 security remediation, invite redesign, per-node identity

Brings in the security remediation branch. Three bodies of work, and what they
changed about what this project may claim.

Phase 11.5 closed the gap between the documents and the code: the unauthenticated
node HTTP API and the TCP transport deleted, one handshake shared by the
remaining two transports, mutual authentication, structured admin transcripts,
upload confinement, group isolation, revocation that reaches nodes. Six critical
and seven high findings closed, bounded, or deferred by decision.

The invite redesign closed H3 and M3 — the last open High. The hub was the key
directory: an inviter fetched the invitee's key from it and wrapped the group key
for whatever came back, so a hub answering with its own key was handed the group
key by an honest member following the protocol exactly. That lookup is gone. The
node holds the group key and wraps it itself, for a key its recipient proves
possession of, bound to an account by a one-time code the hub never sees. M3 fell
out of the same work: node authority comes from a local roster, never from the
hub.

Per-node identity cut what remains of C4 down to one operator. A single keypair
used to be copied to every node its owner joined; each node now gets its own, so
cracking the bundle on one machine yields a key that is a stranger everywhere
else — and on that machine, one that unlocks nothing its holder did not already
serve. The bundle KDF moved to Argon2id 128 MB, and the hub stopped storing or
publishing user keys at all.

What this project may now say: the hub cannot read your content unless it ships
you malicious client code. T3 remains, accepted (D1), and is what the native
client removes. C4 is reduced, not closed, until 13.3. Chat is still plaintext at
rest until Phase 15. Draft-v5 §2 states each claim against the adversary it holds
against, which is the convention this branch exists to keep.

Four defects were found by deploying it and using a browser, none by the test
suite: a node going deaf on its hub socket, a token that predated group
membership, a client reading values before they were assigned, and identity keys
a browser held but never re-read. The lessons are recorded in CLAUDE.md.

Tests: 343 across the three packages, plus QE/deploy/e2e.py — register, pair,
invite, join, download, stream, second browser, revoke — run against the live
deployment on a wiped hub and node.

1 files changed, 53 insertions, 3 deletions

diff --git a/packages/meshbay-hub/src/meshbay_hub/api/signaling.py b/packages/meshbay-hub/src/meshbay_hub/api/signaling.py
index bd343c9..8f84163 100644
--- a/packages/meshbay-hub/src/meshbay_hub/api/signaling.py
+++ b/packages/meshbay-hub/src/meshbay_hub/api/signaling.py
@@ -17,11 +17,15 @@ import json
 import logging
 import uuid
 
-from fastapi import APIRouter, Depends, HTTPException
+from fastapi import APIRouter, Depends, HTTPException, Request
 from pydantic import BaseModel
+from sqlalchemy import select
+from sqlalchemy.ext.asyncio import AsyncSession
 
 from meshbay_hub.api.deps import get_current_user
-from meshbay_hub.db.models import User
+from meshbay_hub.api.middleware import limiter
+from meshbay_hub.db.engine import get_db
+from meshbay_hub.db.models import Group, GroupMember, User
 
 log = logging.getLogger(__name__)
 
@@ -41,25 +45,66 @@ class WebRTCOfferResponse(BaseModel):
     peer_id: str
 
 
+MAX_SDP_BYTES = 16 * 1024          # an SDP offer is ~2 KB
+MAX_PENDING_PER_USER = 3           # concurrent in-flight offers per account
+
+_pending_per_user: dict[str, int] = {}
+
+
 @router.post("/{node_id}/webrtc/offer", response_model=WebRTCOfferResponse)
+@limiter.limit("30/minute")
 async def webrtc_offer(
     node_id: str,
     body: WebRTCOfferRequest,
+    request: Request,
     current_user: User = Depends(get_current_user),
+    db: AsyncSession = Depends(get_db),
 ):
     """
     Browser sends WebRTC SDP offer for a node. Hub relays via WebSocket.
     Returns the node's SDP answer once received.
+
+    Finding H6: this was reachable by any authenticated user, for any node, with no
+    rate limit and no membership check. Each call makes the node allocate an
+    aiortc RTCPeerConnection and gather ICE, so it was a remote resource-exhaustion
+    primitive against an arbitrary third party's machine.
+
+    Finding H4: it also ignored group status, so "suspend a group" did not stop new
+    connections from being brokered to nodes hosting it.
     """
-    from meshbay_hub.api.revocation import _connected_nodes
+    from meshbay_hub.api.revocation import _connected_nodes, _node_groups
+
+    if len(body.sdp) > MAX_SDP_BYTES:
+        raise HTTPException(status_code=413, detail="SDP too large")
 
     ws = _connected_nodes.get(node_id)
     if not ws:
         raise HTTPException(status_code=404, detail="Node not connected")
 
+    # The caller must share at least one active group with the target node.
+    node_group_ids = set(_node_groups.get(node_id, []))
+    if node_group_ids:
+        result = await db.execute(
+            select(GroupMember.group_id).where(
+                GroupMember.user_id == current_user.id,
+                GroupMember.group_id.in_(node_group_ids),
+            ))
+        shared = [gid for (gid,) in result.all()]
+        if not shared:
+            raise HTTPException(status_code=403, detail="Not a member of any group on this node")
+
+        active = await db.execute(
+            select(Group.id).where(Group.id.in_(shared), Group.status == "active"))
+        if not active.first():
+            raise HTTPException(status_code=403, detail="Group is not active")
+
+    if _pending_per_user.get(current_user.id, 0) >= MAX_PENDING_PER_USER:
+        raise HTTPException(status_code=429, detail="Too many pending connections")
+
     peer_id = str(uuid.uuid4())
     answer_future: asyncio.Future = asyncio.get_event_loop().create_future()
     _webrtc_answers[peer_id] = answer_future
+    _pending_per_user[current_user.id] = _pending_per_user.get(current_user.id, 0) + 1
 
     try:
         await ws.send_text(json.dumps({
@@ -83,6 +128,11 @@ async def webrtc_offer(
         )
     finally:
         _webrtc_answers.pop(peer_id, None)
+        remaining = _pending_per_user.get(current_user.id, 1) - 1
+        if remaining > 0:
+            _pending_per_user[current_user.id] = remaining
+        else:
+            _pending_per_user.pop(current_user.id, None)
 
 
 def handle_webrtc_answer(msg: dict) -> None: