feat: Crawl History & Snapshot Comparison View

[saurabhsharma2u]

This PR introduces a comprehensive Crawl History feature to the Crawlith dashboard.
It allows users to:

1. View a Timeline: See all past crawls with key metrics (Pages, Health, Issues).
2. Analyze Trends: Visualize how Total Pages, Health Score, and Broken Links evolve over time with interactive charts.
3. Compare Snapshots: Select any two snapshots to see a detailed diff:
   • Pages Added / Removed
   • Health Score Delta
   • New vs. Resolved Critical Issues (e.g., Broken Links)
4. Manage History: Delete old snapshots with a confirmation.

Technical Changes:

• Core: Enhanced SnapshotRepository.deleteSnapshot to cascade delete pages that are no longer referenced by any snapshot, preventing database bloat.
• Server: Added API endpoints:
  • GET /api/history: List of snapshots.
  • GET /api/history/trends: Time-series data for charts.
  • GET /api/history/compare: Logic to compute diffs between two snapshots using SQL (identifying new/resolved issues).
  • DELETE /api/history/:id: endpoint to remove snapshots.
• Web:
  • New HistoryView container.
  • TrendChart component (lightweight SVG implementation).
  • SnapshotTable with Compare/Delete actions.
  • ComparisonView for detailed diff analysis.
  • Integrated into Sidebar and App routing state.

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PR created automatically by Jules for task 4350587724974819289 started by @saurabhsharma2u

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