Update architecture diagram to include the /dls filesystem mount

[vredchenko]

Description

The architecture diagram should show the DLS shared filesystem (/dls) as a component, since it's now load-bearing for image serving:

• The agent ingests EPU output from the workstation filesystem (and records /dls/... paths into the DB).
• The backend API (image-serving endpoints) reads atlas/grid-square files back off /dls to serve them to the frontend.
• In local dev this is a hostPath mount of testdata/dls-filesystem (devtools PR feat: mount local DLS-filesystem mirror into dev backend for image serving #223); in stage/prod it's the real DLS NFS/PVC (devtools PR docs: DLS filesystem mount placeholder for prod/stage API #224).

Scope

• Add /dls filesystem node + the agent-ingest and API-read edges to the architecture diagram in docs/decision-records/backend-agent-communication-system-design.md.
• Keep the webui mirror in sync (webui/src/docs/decision-records/backend-agent-communication-system-design.mdx).

Notes

Captured during the image-path-resolution work (2026-06-08). Lower priority than the functional changes; deferred deliberately.