feat: Pipelines page — CodePipeline runs and deployment history

[AhmadHammad21]

What

Implement the grayed-out "Pipelines" nav item in the sidebar.

Why

Currently disabled with no explanation. Would surface CodePipeline runs and deployment history from CloudTrail — useful context during incident investigations.

How

• Backend: src/api/routers/pipelines.py — list CodePipeline pipelines, execution history, stage status
• Frontend: frontend/src/pages/PipelinesPage.tsx — table of pipelines with last run status, duration, trigger
• Enable the nav item in Sidebar.tsx

Files

• src/api/routers/pipelines.py (new)
• frontend/src/pages/PipelinesPage.tsx (new)
• frontend/src/components/Sidebar.tsx