feat(hooks): real-time task updates with SSE fallback polling (#233)

[Shikhar-404exe]

Summary #233

Adds a reusable useTaskSubscription hook that provides real-time task status and output
updates via Server-Sent Events (SSE), with automatic polling fallback when the SSE connection
drops. Replaces the inline EventSource logic in TaskDetails.tsx.

Changes

New: useTaskSubscription hook (frontend/src/hooks/useTaskSubscription.ts)

• Connects to GET /api/v1/task/{task_id}/stream via EventSource
• Listens for status and output events, deduplicating identical values
• On SSE error: exponential backoff reconnect (1s, 2s, 4s, 8s, 16s — max 5 attempts),
  then falls back to polling getTaskStatus() at a configurable interval
• On terminal status (completed/failed/cancelled): cleans up EventSource, timers, and polling
• Returns { isConnected, isPolling, error } for UI indicators

Refactored: TaskDetails.tsx

• Inline EventSource setup removed; replaced with useTaskSubscription
• loadTask() is called on mount and when terminal status arrives

Tests (useTaskSubscription.test.ts)

• SSE connection on mount, status/output event callbacks
• Deduplication of identical status values and output chunks
• Reconnect with exponential backoff and max attempt limit
• Polling fallback after reconnect exhaustion
• Polling at configured interval
• Stop polling on terminal status
• Cleanup on unmount

Key design decisions

• reconnectAttemptRef is only reset on onopen (successful connection), not inside
  connectSSE(), so consecutive reconnect failures properly accumulate toward the max limit
• Ref-based callbacks (onStatusRef, onOutputRef) avoid stale closures
• cleanupRef gate prevents race conditions during teardown
• Polling uses setInterval with cleanup checks both before and after the async fetch

[utksh1]

Thanks for extracting the task subscription hook. This needs one correctness fix before merge: deduplicating output by identical chunk content can drop legitimate repeated scanner output lines. Please preserve output stream order exactly, only dedupe status/phase if needed, and add a regression where two identical output chunks are both appended.