feat: add healthcheck endpoint

[fank]

Summary

• Add GET /healthcheck endpoint at root level (outside prefix URL)
• Returns {"status": "ok"} with HTTP 200
• Enables Docker healthcheck and external monitoring

Test plan

• Build succeeds
• Existing tests pass
• Manual test: curl http://localhost:5000/healthcheck returns {"status":"ok"}

[github-actions]

Merging this branch will decrease overall coverage

Impacted Packages	Coverage Δ	🤖
github.com/OCAP2/web/internal/server	44.06% (-0.11%)	👎

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Coverage by file

Changed files (no unit tests)

Changed File	Coverage Δ	Total	Covered	Missed	🤖
github.com/OCAP2/web/internal/server/handler.go	39.67% (-0.25%)	731 (+7)	290 (+1)	441 (+6)	👎

Please note that the "Total", "Covered", and "Missed" counts above refer to code statements instead of lines of code. The value in brackets refers to the test coverage of that file in the old version of the code.