Add PostHog flag handling and team-dependent re-evaluation logic

[cstns]

Description

Fix PostHog stale cache on hard reload when loading feature flags, caused by PostHog being initialized before the team was loaded.

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 76.08%. Comparing base (3081607) to head (528befd).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #8267   +/-   ##
=======================================
  Coverage   76.08%   76.08%           
=======================================
  Files         445      445           
  Lines       23994    23994           
  Branches     6405     6405           
=======================================
  Hits        18255    18255           
  Misses       5739     5739           

Flag	Coverage Δ
backend	76.08% <ø> (ø)

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