feat(project-tree): persist unfiled

[mariusandra]

Problem

We're recalculating the unfiled items every time the tree loads

Changes

• Persist items once, only create new items.
• Support slashes in filenames

Ideally we'd precalculate this for teams (not on first load like now), but this is good enough for testing.

How did you test this code?

Updated tests, clicked around locally.

[greptile-apps]

PR Summary

This PR changes the file system handling to persist unfiled items in the database instead of recalculating them on each tree load.

• Replaced UnfiledFileFinder with UnfiledFileSaver class in /posthog/models/file_system.py to save items to database rather than just retrieving them
• Implemented _generate_unique_path method to handle naming collisions when creating file paths
• Added database queries with Exists and OuterRef to check if items are already saved before creating new entries
• Updated API endpoint in /posthog/api/file_system.py to use save_unfiled_files instead of get_unfiled_files
• Added comprehensive tests for idempotency to ensure duplicate entries aren't created when function is called multiple times

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[github-actions]

Size Change: 0 B

Total Size: 9.72 MB

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Filename	Size
frontend/dist/toolbar.js	9.72 MB

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[posthog-bot]

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1 snapshot changes in total. 0 added, 1 modified, 0 deleted:

• chromium: 0 added, 1 modified, 0 deleted (diff for shard 2)
• webkit: 0 added, 0 modified, 0 deleted

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