feat: replace UI polling with real-time SSE

[roziscoding]

What does this PR do?

Replaces the management UI's HTTP polling with real-time Server-Sent Events while keeping the existing snapshot payloads.

• publishes download snapshots immediately after persisted download mutations
• publishes overview/config snapshots immediately after connector and config changes, including rollback completion
• adds authenticated SSE routes for overview, downloads, and connector config
• uses a simple periodic heartbeat independent from data delivery
• bounds pending writes and disconnects slow consumers so EventSource can reconnect with a fresh snapshot
• shares browser EventSource connections per endpoint and closes them on unmount/logout
• preserves SSE streaming headers and cancellation through the Nuxt BFF
• removes the obsolete polling composables and refresh controls

Related issue

None.

Verification

• mise run lint:fix
• mise run test — 452 passed
• mise run typecheck
• bun run build in apps/ui

Checklist

• Commits follow Conventional Commits
• Commits are signed (GitHub enforces this — unsigned commits are rejected)
• mise run lint:fix was run and its output is committed
• mise run test passes (transfer/request paths were not changed)
• Docs are not required: no configuration or public user workflow changed

[greptile-apps]

Greptile Summary

Replaces management UI polling with authenticated real-time SSE streams.

• Adds backend snapshot publishers for download, connector, and configuration mutations.
• Adds bounded SSE delivery with heartbeats, cancellation, and slow-consumer disconnection.
• Shares browser streams by endpoint and preserves streaming behavior through the Nuxt proxy.
• Removes polling controls and updates dashboard, downloads, and settings views to consume snapshots.

Confidence Score: 5/5

The PR appears safe to merge.

No blocking failure remains.

_{Reviews (2): Last reviewed commit: "fix(ui): handle expired SSE sessions" | Re-trigger Greptile}