feat(#17): web Bifrost-bind — server side (slice 3c, INV-008 lockstep complete)

Slice 3c of issue #17 — the web surface of the bind trigger, server side. Closes
the INV-008 lockstep (CLI + TUI + web all carry the bind now). Implements the
contract's "web bind split": the browser selects only the PLANE; the consumer key
and the Worldtree-visible host are SERVER-HELD config and never reach the browser.

- create_app gains bifrost_consumer_key + bifrost_visible_host (server-held,
  from env via the entrypoint: RATATOSKR_BIFROST_CONSUMER_KEY /
  RATATOSKR_PROVIDER_VISIBLE_HOST).
- _create_session_endpoint reads an optional `bifrost_plane` from the browser
  body, builds the BifrostBinding SERVER-SIDE via endpoint_for_plane(plane,
  visible_host), and calls create_session(bifrost=, consumer_key=). The 201
  response echoes bound-state {plane, endpoint, status: bound} for the UI
  indicator — never the key (INV-008/INV-009).
- Error routing: invalid plane / unconfigured server -> 400; BifrostHandshakeFailed
  -> 502 {bifrost_error}; BifrostConsumerKeyMissing (server misconfig) -> 400.

5 new web bind tests (server constructs binding + key-never-leaks + upstream
carries bifrost body + consumer-key bearer; unconfigured -> 400; invalid plane;
handshake 502; no-plane unbound regression). Full suite 470 green; added lines
ruff + mypy clean (pre-existing web-file backlog untouched).

Follow-on: the index.html plane selector (UI trigger) — the server capability is
complete and TDD'd; the browser-side dropdown is a thin separate change.

LIVE-SMOKE PROVEN (this session): the CLI bind drove a bound sindra session
against personal Worldtree :8081 -> handshake 200 -> the op-feed captured 2
recall searches correlated to the EXACT bound session_id (2c0c7482), with the
real #297/#298 union-recall scopes. Bind + observe proven end-to-end live.

tests/test_web_server.py

@@ -792,3 +792,113 @@ class TestDisconnectCancel:
             await asyncio.sleep(0.02)
         gate.set()
         assert cancel_route.called, "browser disconnect must cancel the UPSTREAM turn (42)"
+
+
+class TestWebBifrostBind:
+    """Issue #17 slice 3c — web bind split: the browser selects the PLANE; the
+    consumer key + visible host are SERVER-HELD and never reach the browser
+    (INV-008/INV-009)."""
+
+    @respx.mock
+    def test_bound_create_server_constructs_binding_key_never_leaks(self) -> None:
+        """tracer: a plane from the browser → the server builds the binding with
+        its OWN consumer key + host, sends the bifrost body + consumer-key bearer
+        upstream, and returns bound-state WITHOUT the key."""
+        import json as _json
+
+        from ratatoskr.web.server import create_app
+
+        route = respx.post("https://w.example/sessions").mock(
+            return_value=httpx.Response(201, json=_CREATE_OK)
+        )
+        app = create_app(
+            _mock_client_factory(),
+            bifrost_consumer_key="server-ck",
+            bifrost_visible_host="10.100.10.50",
+        )
+        resp = TestClient(app).post(
+            "/api/sessions", json={"agent_id": "ratatoskr:sindra", "bifrost_plane": "memory"}
+        )
+        assert resp.status_code == 201
+        # bound-state echoed for the UI indicator — plane + endpoint, NO key
+        assert resp.json()["bifrost"] == {
+            "plane": "memory",
+            "endpoint": "http://10.100.10.50:8391",
+            "status": "bound",
+        }
+        assert "server-ck" not in resp.text  # the key never reaches the browser
+        # upstream got the bifrost body + the consumer-key bearer override
+        upstream = route.calls[0].request
+        body = _json.loads(upstream.content)
+        assert body["bifrost"] == {
+            "endpoint_url": "http://10.100.10.50:8391", "scope": None
+        }
+        assert upstream.headers["Authorization"] == "Bearer server-ck"
+
+    @respx.mock
+    def test_plane_without_server_config_is_400(self) -> None:
+        """A plane requested but no server-held key/host → bifrost_not_configured."""
+        from ratatoskr.web.server import create_app
+
+        app = create_app(_mock_client_factory())  # no bifrost config
+        resp = TestClient(app).post(
+            "/api/sessions", json={"agent_id": "a", "bifrost_plane": "memory"}
+        )
+        assert resp.status_code == 400
+        assert resp.json()["error_code"] == "bifrost_not_configured"
+
+    def test_invalid_plane_is_400(self) -> None:
+        from ratatoskr.web.server import create_app
+
+        app = create_app(
+            _mock_client_factory(),
+            bifrost_consumer_key="ck",
+            bifrost_visible_host="h",
+        )
+        resp = TestClient(app).post(
+            "/api/sessions", json={"agent_id": "a", "bifrost_plane": "persona"}
+        )
+        assert resp.status_code == 400
+        assert resp.json()["error_code"] == "invalid_bifrost_plane"
+
+    @respx.mock
+    def test_handshake_failure_is_502(self) -> None:
+        from ratatoskr.web.server import create_app
+
+        respx.post("https://w.example/sessions").mock(
+            return_value=httpx.Response(
+                502,
+                json={
+                    "error_code": "bifrost_handshake_failed",
+                    "detail": {"bifrost_error": "bifrost.auth_rejected"},
+                },
+            )
+        )
+        app = create_app(
+            _mock_client_factory(),
+            bifrost_consumer_key="ck",
+            bifrost_visible_host="h",
+        )
+        resp = TestClient(app).post(
+            "/api/sessions", json={"agent_id": "a", "bifrost_plane": "memory"}
+        )
+        assert resp.status_code == 502
+        assert resp.json()["error_code"] == "bifrost_handshake_failed"
+        assert resp.json()["bifrost_error"] == "bifrost.auth_rejected"
+
+    @respx.mock
+    def test_no_plane_is_unbound_no_bifrost_in_response(self) -> None:
+        """regression: no bifrost_plane → pre-#17 unbound create, no bifrost key."""
+        from ratatoskr.web.server import create_app
+
+        respx.post("https://w.example/sessions").mock(
+            return_value=httpx.Response(201, json=_CREATE_OK)
+        )
+        app = create_app(
+            _mock_client_factory(),
+            bifrost_consumer_key="ck",
+            bifrost_visible_host="h",
+        )
+        resp = TestClient(app).post("/api/sessions", json={"agent_id": "mimir"})
+        assert resp.status_code == 201
+        assert "bifrost" not in resp.json()