feat(integration): add Notion session export feature

[Aspirant200715]

Implements backend endpoint for syncing Execra sessions to Notion pages. Includes UI modal for export and updates README documentation.

🔗 Related Issue

Closes #70

Note: All PRs must be linked to an open issue. If there is no issue, create one first.

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📝 Summary of Changes

This PR introduces a brand-new Notion integration allowing users to export their recorded Execra sessions directly to a Notion page.

• Backend: Added the notion-client dependency. Created NotionExporter which formats actions into standard 100-row Notion table blocks to avoid rate-limits. Added a new endpoint POST /api/v1/session/export/notion.
• Frontend: Integrated a new "Export to Notion" button in the SvelteKit dashboard under ADMIN_CONTROLS. Replaced generic browser alerts with a polished, custom success/error modal overlay containing the generated Notion page link.
• Documentation: Added detailed Notion setup instructions to README.md and added a note directing users to read CONTRIBUTING.md.
• Testing: Added robust unit/integration tests for both the exporter logic and the API endpoint.

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🔍 Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
• 📖 Documentation update
• 🧪 Test addition or improvement
• 🧹 Refactor / Code cleanup (no functional change)
• ⚙️ Build / CI / Chore

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🧪 How Was This Tested?

# Executed all backend tests to ensure the exporter and API routes work and handle edge cases
python -m pytest tests/api/test_session_api.py -v
python -m pytest tests/integrations/test_notion_exporter.py -v

# Built the SvelteKit frontend to guarantee zero compilation errors
cd dashboard && npm run build