Per-workspace queue routing for workspace-scoped services (#862)

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, knowledge cores, and config-svc
- 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
- Remove workspace field from request schemas (FlowRequest,
  LibrarianRequest, KnowledgeRequest, CollectionManagementRequest) and
  from DocumentMetadata / ProcessingMetadata — table stores now accept
  workspace as an explicit parameter
- Strip workspace encode/decode from all message translators and gateway
  serializers
- Gateway enforces workspace existence: reject requests targeting
  non-existent workspaces instead of routing to queues with no consumer
- Config service provisions new workspaces from __template__ on creation
- Add workspace lifecycle hooks to AsyncProcessor so any processor can
  react to workspace create/delete without subclassing WorkspaceProcessor

tests/unit/test_translators/test_knowledge_translator_roundtrip.py

@@ -41,7 +41,6 @@ def translator():
 def graph_embeddings_request():
     return KnowledgeRequest(
         operation="put-kg-core",
-        workspace="alice",
         id="doc-1",
         flow="default",
         collection="testcoll",
@@ -110,7 +109,7 @@ class TestKnowledgeRequestTranslatorGraphEmbeddings:
 
         assert isinstance(decoded, KnowledgeRequest)
         assert decoded.operation == "put-kg-core"
-        assert decoded.workspace == "alice"
+        assert decoded.id == "doc-1"
         assert decoded.id == "doc-1"
         assert decoded.flow == "default"
         assert decoded.collection == "testcoll"