feat(agent): add read-only node logs endpoint

[Pranav-d33]

Implements GET /api/v1/nodes/logs?type=PEER&name=&tail= on the Fabric agent, prerequisite for copilot fault-log summarization. Uses docker logs with bounded tail (1..1000, default 200) and maps NotFound to 404. Adds serializer validation and view tests.

Part of #813 — prerequisite for @agamatlab's fault-log summarization tool (#3 in the
work split).

What this does

Adds GET /api/v1/nodes/logs?type=PEER&name=<name>&tail=<n> to the Fabric agent
(src/agents/hyperledger-fabric/). Returns the last N lines of Docker container logs
for a given node.

• get_node_logs() in service.py — same pattern as the existing
  get_node_status(): reads CRYPTO_CONFIG YAML, resolves container name from org
  domain, calls container.logs(tail=tail)
• NodeLogsRequestSerializer — validates type (PEER/ORDERER), name, and tail
  (1–1000, default 200)
• NodeLogsResponseSerializer — { logs: str }
• NodeViewSet.logs action — @action(detail=False, methods=['get'], url_path='logs'), maps docker.errors.NotFound to 404
• 7 tests covering the service function, serializer bounds, and view responses
  (200/400/404)

Why separate PR

The logs endpoint is a Fabric agent change with no overlap with the dashboard panel
or the copilot endpoint. Keeping it separate so it can be reviewed and merged
independently without waiting on the larger copilot pieces.
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