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/schema/services/library.py

@@ -24,10 +24,13 @@ from ..core.metadata import Metadata
 #   <- (document_metadata)
 #   <- (error)
 
-# get-document-content
+# get-document-content [DEPRECATED — use stream-document instead]
 #   -> (document_id)
 #   <- (content)
 #   <- (error)
+#   NOTE: Returns entire document in a single message. Fails for documents
+#   exceeding the broker's max message size. Use stream-document which
+#   returns content in chunks.
 
 # add-processing
 #   -> (processing_id, processing_metadata)
@@ -220,5 +223,5 @@ class LibrarianResponse:
 # FIXME: Is this right?  Using persistence on librarian so that
 # message chunking works
 
-librarian_request_queue = queue('librarian-request', cls='flow')
-librarian_response_queue = queue('librarian-response', cls='flow')
+librarian_request_queue = queue('librarian', cls='request')
+librarian_response_queue = queue('librarian', cls='response')