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

specs/api/components/schemas/librarian/LibrarianRequest.yaml

@@ -3,6 +3,9 @@ description: |
   Librarian service request for document library management.
 
   Operations: add-document, remove-document, list-documents,
+  get-document-metadata, stream-document, add-child-document,
+  list-children, begin-upload, upload-chunk, complete-upload,
+  abort-upload, get-upload-status, list-uploads,
   start-processing, stop-processing, list-processing
 required:
   - operation
@@ -13,6 +16,17 @@ properties:
       - add-document
       - remove-document
       - list-documents
+      - get-document-metadata
+      - get-document-content
+      - stream-document
+      - add-child-document
+      - list-children
+      - begin-upload
+      - upload-chunk
+      - complete-upload
+      - abort-upload
+      - get-upload-status
+      - list-uploads
       - start-processing
       - stop-processing
       - list-processing
@@ -21,6 +35,21 @@ properties:
       - `add-document`: Add document to library
       - `remove-document`: Remove document from library
       - `list-documents`: List documents in library
+      - `get-document-metadata`: Get document metadata
+      - `get-document-content`: Get full document content in a single response.
+        **Deprecated** — use `stream-document` instead. Fails for documents
+        exceeding the broker's max message size.
+      - `stream-document`: Stream document content in chunks. Each response
+        includes `chunk_index` and `is_final`. Preferred over `get-document-content`
+        for all document sizes.
+      - `add-child-document`: Add a child document (e.g. page, chunk)
+      - `list-children`: List child documents of a parent
+      - `begin-upload`: Start a chunked upload session
+      - `upload-chunk`: Upload a chunk of data
+      - `complete-upload`: Finalize a chunked upload
+      - `abort-upload`: Cancel a chunked upload
+      - `get-upload-status`: Check upload progress
+      - `list-uploads`: List active upload sessions
       - `start-processing`: Start processing library documents
       - `stop-processing`: Stop library processing
       - `list-processing`: List processing status