Add tab presence support to track user activity across sessions

[cstns]

Description

• Add MQTT-based browser session presence so third-party MCP agents can discover which tabs a user has exposed
• Forge stores presence in a Redis cache bucket (browserSessions) with a 135s TTL, auto-expired when heartbeats stop
• Browser tabs publish on ff/v1/tab-presence///heartbeat and .../context using the existing fe-team MQTT connection
• Forge subscribes via a shared subscription ($share/platform/...) so only one replica handles each message, avoiding redundant Redis writes

The browser sends two types of messages over MQTT:

• Heartbeat (periodic, ~45s): extends TTL, carries visibility/focus state
• Context (on route change): carries the full contextStore.expert payload so the gateway can reconstitute in-flight MQTT topics with the correct entity info

Both message types call cache.set() which resets the TTL. If heartbeats stop (browser crash, tab close), the entry auto-expires after ~135s.

app.comms.browserSessions.getSessionsByUser(userId) retrieves all active sessions for a user, for the future list_browser_sessions MCP tool (#8161).

Related Issue(s)

closes #8158

Checklist

• I have read the contribution guidelines
• Suitable unit/system level tests have been added and they pass
• Documentation has been updated
  • Upgrade instructions
  • Configuration details
  • Concepts
• Changes flowforge.yml?
  • Issue/PR raised on FlowFuse/helm to update ConfigMap Template
  • Issue/PR raised on FlowFuse/CloudProject to update values for Staging/Production
• Link to Changelog Entry PR, or note why one is not needed.

Labels

• Includes a DB migration? -> add the area:migration label

[codecov]

Codecov Report

❌ Patch coverage is 75.47170% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.19%. Comparing base (2af687c) to head (bc151d0).

Files with missing lines	Patch %	Lines
forge/comms/commsClient.js	0.00%	10 Missing ⚠️
forge/comms/aclManager.js	75.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8182      +/-   ##
==========================================
- Coverage   76.19%   76.19%   -0.01%     
==========================================
  Files         441      442       +1     
  Lines       23619    23672      +53     
  Branches     6289     6298       +9     
==========================================
+ Hits        17996    18036      +40     
- Misses       5623     5636      +13     

Flag	Coverage Δ
backend	76.19% <75.47%> (-0.01%)	⬇️

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