Fix memory leak in /ai/analyze_stream SSE endpoint (#3949)

[singhanurag0317-bit]

Changes

• Added
  equest: Request\ parameter to \�nalyze_stream\ endpoint to detect client disconnects
• Added \�wait request.is_disconnected()\ checks after each yield point — returns early if client is gone, preventing orphaned background processing
• Wrapped blocking Gemini API calls (\�nalyze_image, \get_summary) in \�syncio.to_thread\ + \�syncio.wait_for\ with 30-second timeout to prevent indefinite blocking
• Added \ ry/except/finally\ cleanup with \CancelledError\ handling in event generator
• Added structured log messages for timeouts, errors, and cleanup events

Problem

When clients disconnect mid-stream (page close, network drop), the SSE handler continued running expensive Gemini API calls in the background with no timeout, causing memory leaks and wasted compute.

Solution

Three-layer fix: (1) check disconnect at each step, (2) timeout Gemini calls at 30s, (3) clean up generator on cancellation.

Closes #3949

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• backend/main.py

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