RabbitMQ pub/sub backend with topic exchange architecture (#752)

Adds a RabbitMQ backend as an alternative to Pulsar, selectable via
PUBSUB_BACKEND=rabbitmq. Both backends implement the same PubSubBackend
protocol — no application code changes needed to switch.

RabbitMQ topology:
- Single topic exchange per topicspace (e.g. 'tg')
- Routing key derived from queue class and topic name
- Shared consumers: named queue bound to exchange (competing, round-robin)
- Exclusive consumers: anonymous auto-delete queue (broadcast, each gets
  every message). Used by Subscriber and config push consumer.
- Thread-local producer connections (pika is not thread-safe)
- Push-based consumption via basic_consume with process_data_events
  for heartbeat processing

Consumer model changes:
- Consumer class creates one backend consumer per concurrent task
  (required for pika thread safety, harmless for Pulsar)
- Consumer class accepts consumer_type parameter
- Subscriber passes consumer_type='exclusive' for broadcast semantics
- Config push consumer uses consumer_type='exclusive' so every
  processor instance receives config updates
- handle_one_from_queue receives consumer as parameter for correct
  per-connection ack/nack

LibrarianClient:
- New shared client class replacing duplicated librarian request-response
  code across 6+ services (chunking, decoders, RAG, etc.)
- Uses stream-document instead of get-document-content for fetching
  document content in 1MB chunks (avoids broker message size limits)
- Standalone object (self.librarian = LibrarianClient(...)) not a mixin
- get-document-content marked deprecated in schema and OpenAPI spec

Serialisation:
- Extracted dataclass_to_dict/dict_to_dataclass to shared
  serialization.py (used by both Pulsar and RabbitMQ backends)

Librarian queues:
- Changed from flow class (persistent) back to request/response class
  now that stream-document eliminates large single messages
- API upload chunk size reduced from 5MB to 3MB to stay under broker
  limits after base64 encoding

Factory and CLI:
- get_pubsub() handles 'rabbitmq' backend with RabbitMQ connection params
- add_pubsub_args() includes RabbitMQ options (host, port, credentials)
- add_pubsub_args(standalone=True) defaults to localhost for CLI tools
- init_trustgraph skips Pulsar admin setup for non-Pulsar backends
- tg-dump-queues and tg-monitor-prompts use backend abstraction
- BaseClient and ConfigClient accept generic pubsub config

trustgraph-base/trustgraph/clients/base.py

@@ -18,9 +18,7 @@ class BaseClient:
             output_queue=None,
             input_schema=None,
             output_schema=None,
-            pulsar_host="pulsar://pulsar:6650",
-            pulsar_api_key=None,
-            listener=None,
+            **pubsub_config,
     ):
 
         if input_queue == None: raise RuntimeError("Need input_queue")
@@ -32,12 +30,7 @@ class BaseClient:
             subscriber = str(uuid.uuid4())
 
         # Create backend using factory
-        self.backend = get_pubsub(
-            pulsar_host=pulsar_host,
-            pulsar_api_key=pulsar_api_key,
-            pulsar_listener=listener,
-            pubsub_backend='pulsar'
-        )
+        self.backend = get_pubsub(**pubsub_config)
 
         self.producer = self.backend.create_producer(
             topic=input_queue,

trustgraph-base/trustgraph/clients/config_client.py

@@ -33,9 +33,7 @@ class ConfigClient(BaseClient):
             subscriber=None,
             input_queue=None,
             output_queue=None,
-            pulsar_host="pulsar://pulsar:6650",
-            listener=None,
-            pulsar_api_key=None,
+            **pubsub_config,
     ):
 
         if input_queue == None:
@@ -48,11 +46,9 @@ class ConfigClient(BaseClient):
             subscriber=subscriber,
             input_queue=input_queue,
             output_queue=output_queue,
-            pulsar_host=pulsar_host,
-            pulsar_api_key=pulsar_api_key,
             input_schema=ConfigRequest,
             output_schema=ConfigResponse,
-            listener=listener,
+            **pubsub_config,
         )
 
     def get(self, keys, timeout=300):