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_decoding/test_universal_processor.py

@@ -142,8 +142,8 @@ class TestPageBasedFormats:
 class TestUniversalProcessor(IsolatedAsyncioTestCase):
     """Test universal decoder processor."""
 
-    @patch('trustgraph.decoding.universal.processor.Consumer')
-    @patch('trustgraph.decoding.universal.processor.Producer')
+    @patch('trustgraph.base.librarian_client.Consumer')
+    @patch('trustgraph.base.librarian_client.Producer')
     @patch('trustgraph.base.async_processor.AsyncProcessor', MockAsyncProcessor)
     async def test_processor_initialization(
         self, mock_producer, mock_consumer
@@ -169,8 +169,8 @@ class TestUniversalProcessor(IsolatedAsyncioTestCase):
         assert consumer_specs[0].name == "input"
         assert consumer_specs[0].schema == Document
 
-    @patch('trustgraph.decoding.universal.processor.Consumer')
-    @patch('trustgraph.decoding.universal.processor.Producer')
+    @patch('trustgraph.base.librarian_client.Consumer')
+    @patch('trustgraph.base.librarian_client.Producer')
     @patch('trustgraph.base.async_processor.AsyncProcessor', MockAsyncProcessor)
     async def test_processor_custom_strategy(
         self, mock_producer, mock_consumer
@@ -188,8 +188,8 @@ class TestUniversalProcessor(IsolatedAsyncioTestCase):
         assert processor.partition_strategy == "hi_res"
         assert processor.section_strategy_name == "heading"
 
-    @patch('trustgraph.decoding.universal.processor.Consumer')
-    @patch('trustgraph.decoding.universal.processor.Producer')
+    @patch('trustgraph.base.librarian_client.Consumer')
+    @patch('trustgraph.base.librarian_client.Producer')
     @patch('trustgraph.base.async_processor.AsyncProcessor', MockAsyncProcessor)
     async def test_group_by_page(self, mock_producer, mock_consumer):
         """Test page grouping of elements."""
@@ -214,8 +214,8 @@ class TestUniversalProcessor(IsolatedAsyncioTestCase):
         assert result[1][0] == 2
         assert len(result[1][1]) == 1
 
-    @patch('trustgraph.decoding.universal.processor.Consumer')
-    @patch('trustgraph.decoding.universal.processor.Producer')
+    @patch('trustgraph.base.librarian_client.Consumer')
+    @patch('trustgraph.base.librarian_client.Producer')
     @patch('trustgraph.decoding.universal.processor.partition')
     @patch('trustgraph.base.async_processor.AsyncProcessor', MockAsyncProcessor)
     async def test_on_message_inline_non_page(
@@ -255,7 +255,7 @@ class TestUniversalProcessor(IsolatedAsyncioTestCase):
         }.get(name))
 
         # Mock save_child_document and magic
-        processor.save_child_document = AsyncMock(return_value="mock-id")
+        processor.librarian.save_child_document = AsyncMock(return_value="mock-id")
 
         with patch('trustgraph.decoding.universal.processor.magic') as mock_magic:
             mock_magic.from_buffer.return_value = "text/markdown"
@@ -271,8 +271,8 @@ class TestUniversalProcessor(IsolatedAsyncioTestCase):
         assert call_args.document_id.startswith("urn:section:")
         assert call_args.text == b""
 
-    @patch('trustgraph.decoding.universal.processor.Consumer')
-    @patch('trustgraph.decoding.universal.processor.Producer')
+    @patch('trustgraph.base.librarian_client.Consumer')
+    @patch('trustgraph.base.librarian_client.Producer')
     @patch('trustgraph.decoding.universal.processor.partition')
     @patch('trustgraph.base.async_processor.AsyncProcessor', MockAsyncProcessor)
     async def test_on_message_page_based(
@@ -310,7 +310,7 @@ class TestUniversalProcessor(IsolatedAsyncioTestCase):
             "triples": mock_triples_flow,
         }.get(name))
 
-        processor.save_child_document = AsyncMock(return_value="mock-id")
+        processor.librarian.save_child_document = AsyncMock(return_value="mock-id")
 
         with patch('trustgraph.decoding.universal.processor.magic') as mock_magic:
             mock_magic.from_buffer.return_value = "application/pdf"
@@ -323,8 +323,8 @@ class TestUniversalProcessor(IsolatedAsyncioTestCase):
         call_args = mock_output_flow.send.call_args_list[0][0][0]
         assert call_args.document_id.startswith("urn:page:")
 
-    @patch('trustgraph.decoding.universal.processor.Consumer')
-    @patch('trustgraph.decoding.universal.processor.Producer')
+    @patch('trustgraph.base.librarian_client.Consumer')
+    @patch('trustgraph.base.librarian_client.Producer')
     @patch('trustgraph.decoding.universal.processor.partition')
     @patch('trustgraph.base.async_processor.AsyncProcessor', MockAsyncProcessor)
     async def test_images_stored_not_emitted(
@@ -361,7 +361,7 @@ class TestUniversalProcessor(IsolatedAsyncioTestCase):
             "triples": mock_triples_flow,
         }.get(name))
 
-        processor.save_child_document = AsyncMock(return_value="mock-id")
+        processor.librarian.save_child_document = AsyncMock(return_value="mock-id")
 
         with patch('trustgraph.decoding.universal.processor.magic') as mock_magic:
             mock_magic.from_buffer.return_value = "application/pdf"
@@ -374,7 +374,7 @@ class TestUniversalProcessor(IsolatedAsyncioTestCase):
         assert mock_triples_flow.send.call_count == 2
 
         # save_child_document called twice (page + image)
-        assert processor.save_child_document.call_count == 2
+        assert processor.librarian.save_child_document.call_count == 2
 
     @patch('trustgraph.base.flow_processor.FlowProcessor.add_args')
     def test_add_args(self, mock_parent_add_args):