perf(project-details): optimize loading and conditional caching [3/5]

[Anthony Cintron (prbatero)]

Summary

Part 3 of 5 in the project layer-loading performance stack. Replace the endpoint’s nested storage reads with tested keyed orchestration and add real conditional response caching.

Changes

• Move project-detail loading and assembly into ProjectDetailsProcessor.
• Preserve legacy artifact and validation joins by storage identifier.
• Batch validation and artifact reads through the PR 2 primitives.
• Add a bounded process-local TTL cache with same-key single-flight behavior.
• Support ETag/304 responses and explicit Cache-Control: no-cache refreshes.
• Hoist label loading in GenerateProjectStats and handle unlabeled layers.
• Expose ETag and performance headers through the local API proxy.

Testing

• 50 applicable API tests passed; two request-boundary tests are owned by independent PR fix(api): reject client-supplied workflow state #184.
• 28 project-details/cache tests passed.
• Black, isort, flake8, and detect-secrets passed.

Stack

• Depends on: perf(data-layer): add bounded batch reads and Blob client reuse [2/5] #186.
• Related independent fix: fix(api): reject client-supplied workflow state #184.
• This PR: 3/5, based on prbatero/feat/perf-data-layer-core.
• Next branch: prbatero/feat/perf-project-details-ui.

Review Notes

The cache is bounded and process-local, not distributed. A fresh matching ETag avoids storage work; after TTL expiry the representation is reloaded before returning 304.