Implement local screenshot preprocessing

[BryanChasko]

Agent Task

Agent: ellow-gander-mechanic
Depends on: none
Branch: feat/snap-preprocessing
Source: specs/marvel-snap-pipeline/tasks.md

Acceptance Criteria

• Crop game UI region from full screenshot
• Strip EXIF metadata (GPS, device info)
• Preserve EXIF timestamp for session grouping
• Output sanitized image ready for external model

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Generated from specs/marvel-snap-pipeline/tasks.md by generate-task-issues.py