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_concurrency/test_consumer_concurrency.py

@@ -83,7 +83,7 @@ class TestTaskGroupConcurrency:
         call_count = 0
         original_running = True
 
-        async def mock_consume():
+        async def mock_consume(backend_consumer):
             nonlocal call_count
             call_count += 1
             # Wait a bit to let all tasks start, then signal stop
@@ -107,7 +107,7 @@ class TestTaskGroupConcurrency:
         consumer = _make_consumer(concurrency=1)
         call_count = 0
 
-        async def mock_consume():
+        async def mock_consume(backend_consumer):
             nonlocal call_count
             call_count += 1
             await asyncio.sleep(0.01)
@@ -147,7 +147,7 @@ class TestRateLimitRetry:
         mock_msg = _make_msg()
         consumer.consumer = MagicMock()
 
-        await consumer.handle_one_from_queue(mock_msg)
+        await consumer.handle_one_from_queue(mock_msg, consumer.consumer)
 
         assert call_count == 2
         consumer.consumer.acknowledge.assert_called_once_with(mock_msg)
@@ -166,7 +166,7 @@ class TestRateLimitRetry:
         mock_msg = _make_msg()
         consumer.consumer = MagicMock()
 
-        await consumer.handle_one_from_queue(mock_msg)
+        await consumer.handle_one_from_queue(mock_msg, consumer.consumer)
 
         consumer.consumer.negative_acknowledge.assert_called_with(mock_msg)
         consumer.consumer.acknowledge.assert_not_called()
@@ -185,7 +185,7 @@ class TestRateLimitRetry:
         mock_msg = _make_msg()
         consumer.consumer = MagicMock()
 
-        await consumer.handle_one_from_queue(mock_msg)
+        await consumer.handle_one_from_queue(mock_msg, consumer.consumer)
 
         assert call_count == 1
         consumer.consumer.negative_acknowledge.assert_called_once_with(mock_msg)
@@ -197,7 +197,7 @@ class TestRateLimitRetry:
         mock_msg = _make_msg()
         consumer.consumer = MagicMock()
 
-        await consumer.handle_one_from_queue(mock_msg)
+        await consumer.handle_one_from_queue(mock_msg, consumer.consumer)
 
         consumer.consumer.acknowledge.assert_called_once_with(mock_msg)
 
@@ -219,7 +219,7 @@ class TestMetricsIntegration:
         mock_metrics.record_time.return_value.__exit__ = MagicMock()
         consumer.metrics = mock_metrics
 
-        await consumer.handle_one_from_queue(mock_msg)
+        await consumer.handle_one_from_queue(mock_msg, consumer.consumer)
 
         mock_metrics.process.assert_called_once_with("success")
 
@@ -235,7 +235,7 @@ class TestMetricsIntegration:
         mock_metrics = MagicMock()
         consumer.metrics = mock_metrics
 
-        await consumer.handle_one_from_queue(mock_msg)
+        await consumer.handle_one_from_queue(mock_msg, consumer.consumer)
 
         mock_metrics.process.assert_called_once_with("error")
 
@@ -261,7 +261,7 @@ class TestMetricsIntegration:
         mock_metrics.record_time.return_value.__exit__ = MagicMock(return_value=False)
         consumer.metrics = mock_metrics
 
-        await consumer.handle_one_from_queue(mock_msg)
+        await consumer.handle_one_from_queue(mock_msg, consumer.consumer)
 
         mock_metrics.rate_limit.assert_called_once()