unify contract event topic naming and stable ID docs for indexer compatibility

[grantfox-oss]

the contract currently defines event topic constants and an event schema list, but backend indexer docs and contract README are out of sync with actual event payload shapes.

• Impacted files: app/contract/contracts/Folder/src/events.rs, app/contract/contracts/Folder/src/lib.rs, app/contract/contracts/Folder/src/test.rs, app/contract/README.md, app/contract/docs/UPGRADE_SAFETY_GATE.md, `app/backend/README.md

[eulami]

Hi. Having reviewed this issue and based on my strong knowledge, I I'm confident of submitting a clean PR in 48 hours. Kindly assign it to me.

[grantfox-oss]

🦊 GrantFox — @eulami has been assigned to this issue as part of the Official Campaign | FWC26 campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #331)
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