Y26-035 - Add health endpoint

[StephenHulme]

Adds a dedicated health endpoint as part of sanger/General-Backlog-Items#700

Changes proposed in this pull request

• Add HealthController and /health endpoint

Instructions for Reviewers

[All PRs] - Confirm PR template filled
[Feature Branches] - Review code
[Production Merges to main]
    - Check story numbers included
    - Check for debug code
    - Check version

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.26%. Comparing base (eb9bda1) to head (c0dc875).
⚠️ Report is 15 commits behind head on develop.

Files with missing lines	Patch %	Lines
app/controllers/health_controller.rb	75.00%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #654      +/-   ##
===========================================
- Coverage    80.31%   80.26%   -0.06%     
===========================================
  Files           74       75       +1     
  Lines         1128     1140      +12     
===========================================
+ Hits           906      915       +9     
- Misses         222      225       +3     

Flag	Coverage Δ
pull_request	80.26% <75.00%> (?)
push	80.26% <75.00%> (-0.06%)	⬇️

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