perf: Add Cache-Control headers to GET endpoints

[smoreinis]

Summary

• Adds reusable @cacheable decorator for HTTP cache headers
• Applies cache headers to 9 GET endpoints across 4 route files
• Reduces server load by enabling client-side caching

Changes

• New src/api/cache.py: Decorator supporting max_age, private/public, stale-while-revalidate
• agents.py: 60s cache for agent data (3 endpoints)
• schedules.py: 60s cache for schedule config (2 endpoints)
• spans.py: 300s cache for historical trace data (2 endpoints)
• deployment_history.py: 300s cache for historical deployments (2 endpoints)

Usage

@router.get("/items/{id}")
@cacheable(max_age=60)
async def get_item(id: str, response: Response) -> Item:
    ...

Test plan

• All existing tests pass (184/184)
• Manual verification: Cache-Control header present

$ curl -i http://localhost:5003/spans 2>&1 | grep -i cache
cache-control: private, max-age=300