Add contextual filtering to the dashboard instances api endpoint

[cstns]

Description

Update the dashboard instances api endpoint to filter out instances based on rbac application context.

I also needed to define a new permission, team:projects:list-dashboards, to check against. This permission (dashboard role) should be lower than the existing team:projects:list (viewer role) to allow dashboard users to list available instances while blocking those where their team application role is None (0).

Related Issue(s)

N/a

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

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.70%. Comparing base (b1bd883) to head (d6a8938).
⚠️ Report is 4 commits behind head on 5745-granular-rbac-backend.

Additional details and impacted files

@@                     Coverage Diff                     @@
##           5745-granular-rbac-backend    #6061   +/-   ##
===========================================================
  Coverage                       76.69%   76.70%           
===========================================================
  Files                             380      380           
  Lines                           19119    19124    +5     
  Branches                         4565     4566    +1     
===========================================================
+ Hits                            14664    14669    +5     
  Misses                           4455     4455           

Flag	Coverage Δ
backend	76.70% <100.00%> (+<0.01%)	⬆️

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