Phase 7: Bot/extension system

Bot accounts (User rows with is_bot=True), scoped API tokens (read:messages,
write:messages, manage:rooms) authenticated via Authorization: Bearer on both
REST and the WS handshake, live bot WebSocket access on the same /ws/chat
endpoint humans use, message editing (WS "edit" envelope -> message_update
broadcast, fans out cross-instance for free via the existing broadcaster),
incoming webhooks (room-scoped, no auth beyond the URL token), and outgoing
webhooks/event subscriptions (HMAC-SHA256 signed, backgrounded delivery,
creation-time SSRF validation against private/loopback/link-local targets).

Token auth is additive, not a parallel system: a bearer-token-authenticated
bot goes through the exact same room-membership/role checks a session-
authenticated human does everywhere; only read:messages/write:messages are
separately scope-gated (the two message endpoints). manage:rooms scope
enforcement, full per-delivery SSRF re-validation, and bot API rate limiting
were explicitly scoped out (confirmed with the repo owner) as disproportionate
to this phase -- documented as known gaps in backend/README.md rather than
silently skipped.

Admin portal gains a Bots tab (create bots, issue/revoke scoped tokens,
cross-room webhook visibility); RoomInfoPanel gains room-scoped webhook/
subscription management, mirroring how invites already work there. The chat
UI also gets a minimal "edit your own message" affordance -- not asked for
by the issue, but the only practical way to exercise the edit pipeline by
hand instead of only via a scripted bot client.

Along the way: fixed a real bug caught while writing the incoming-webhook
test -- offline-push notification relied on the sender being "connected" to
exclude themselves, true for WS-originated messages but not for the new
webhook path, which has no WS connection for the attributed sender at all.
Now explicitly excluded. Also discovered the REST-only test fixture never
triggered ASGI lifespan, so app.state.broadcaster/presence didn't exist for
it; moved their construction out of the lifespan into create_app() itself
(Redis client construction is synchronous/lazy) so both the WS and
REST-only paths always have them.

New tests/test_bots.py, test_message_edit.py, test_webhooks.py (full suite
now 78/78, stable across repeated runs) plus a scripted end-to-end smoke
test (bot WS join/post/edit, incoming webhook, SSRF rejection, outgoing
delivery) and a full browser walkthrough of the new admin/room UI.

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

backend/app/services/ssrf.py

@@ -0,0 +1,38 @@
+import ipaddress
+import socket
+from urllib.parse import urlparse
+
+
+class UnsafeWebhookUrlError(Exception):
+    pass
+
+
+def validate_target_url(url: str) -> None:
+    """Creation-time-only SSRF check: rejects non-http(s) schemes and any
+    target whose hostname resolves to a private/loopback/link-local/
+    reserved/multicast address. Not re-checked per delivery, so this doesn't
+    defend against DNS rebinding between creation and a later send -- a
+    documented known limitation, not an oversight.
+    """
+    parsed = urlparse(url)
+    if parsed.scheme not in ("http", "https"):
+        raise UnsafeWebhookUrlError()
+    if not parsed.hostname:
+        raise UnsafeWebhookUrlError()
+
+    try:
+        addrinfo = socket.getaddrinfo(parsed.hostname, None)
+    except socket.gaierror as exc:
+        raise UnsafeWebhookUrlError() from exc
+
+    for *_rest, sockaddr in addrinfo:
+        ip = ipaddress.ip_address(sockaddr[0])
+        if (
+            ip.is_private
+            or ip.is_loopback
+            or ip.is_link_local
+            or ip.is_reserved
+            or ip.is_multicast
+            or ip.is_unspecified
+        ):
+            raise UnsafeWebhookUrlError()