Per-workspace queue routing for workspace-scoped services

Workspace identity is now determined by queue infrastructure instead of
message body fields, closing a privilege-escalation vector where a caller
could spoof workspace in the request payload.

- Add WorkspaceProcessor base class: discovers workspaces from config at
  startup, creates per-workspace consumers (queue:workspace), and manages
  consumer lifecycle on workspace create/delete events
- Roll out to librarian, flow-svc, and knowledge cores
- Remove workspace field from request schemas (FlowRequest,
  LibrarianRequest, KnowledgeRequest, CollectionManagementRequest) and
  from DocumentMetadata / ProcessingMetadata — table stores now accept
  workspace as an explicit parameter for Cassandra partition keys
- Strip workspace encode/decode from all message translators and gateway
  serializers
- Config service gets a dual-queue regime: a system queue for
  cross-workspace ops (getvalues-all-ws, bootstrapper writes to
  __workspaces__) and per-workspace queues for tenant-scoped ops, with
  workspace discovery from its own Cassandra store
- Gateway enforces workspace requirement for workspace dispatchers —
  config moves from system_dispatchers to workspace_dispatchers so the
  gateway can never route to the system config queue
- Add workspace lifecycle hooks to AsyncProcessor so any processor can
  react to workspace create/delete without subclassing WorkspaceProcessor

tests/unit/test_base/test_flow_processor.py

@@ -233,7 +233,7 @@ class TestFlowProcessorSimple(IsolatedAsyncioTestCase):
         mock_flow2.start.assert_called_once()
 
     @with_async_processor_patches
-    @patch('trustgraph.base.async_processor.AsyncProcessor.start')
+    @patch('trustgraph.base.workspace_processor.WorkspaceProcessor.start')
     async def test_start_calls_parent(self, mock_parent_start, *mocks):
         """Test that start() calls parent start method"""
         mock_parent_start.return_value = None