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

tests/unit/test_pubsub/test_queue_naming.py

@@ -109,6 +109,37 @@ class TestAddPubsubArgs:
         assert args.pubsub_backend == 'pulsar'
 
 
+class TestAddPubsubArgsRabbitMQ:
+
+    def test_rabbitmq_args_present(self):
+        parser = argparse.ArgumentParser()
+        add_pubsub_args(parser)
+        args = parser.parse_args([
+            '--pubsub-backend', 'rabbitmq',
+            '--rabbitmq-host', 'myhost',
+            '--rabbitmq-port', '5673',
+        ])
+        assert args.pubsub_backend == 'rabbitmq'
+        assert args.rabbitmq_host == 'myhost'
+        assert args.rabbitmq_port == 5673
+
+    def test_rabbitmq_defaults_container(self):
+        parser = argparse.ArgumentParser()
+        add_pubsub_args(parser)
+        args = parser.parse_args([])
+        assert args.rabbitmq_host == 'rabbitmq'
+        assert args.rabbitmq_port == 5672
+        assert args.rabbitmq_username == 'guest'
+        assert args.rabbitmq_password == 'guest'
+        assert args.rabbitmq_vhost == '/'
+
+    def test_rabbitmq_standalone_defaults_to_localhost(self):
+        parser = argparse.ArgumentParser()
+        add_pubsub_args(parser, standalone=True)
+        args = parser.parse_args([])
+        assert args.rabbitmq_host == 'localhost'
+
+
 class TestQueueDefinitions:
     """Verify the actual queue constants produce correct names."""
 
@@ -124,9 +155,9 @@ class TestQueueDefinitions:
         from trustgraph.schema.services.config import config_push_queue
         assert config_push_queue == 'state:tg:config'
 
-    def test_librarian_request_is_persistent(self):
+    def test_librarian_request(self):
         from trustgraph.schema.services.library import librarian_request_queue
-        assert librarian_request_queue.startswith('flow:')
+        assert librarian_request_queue == 'request:tg:librarian'
 
     def test_knowledge_request(self):
         from trustgraph.schema.knowledge.knowledge import knowledge_request_queue