feat: apply media attachment decay settings retroactively

[SudhanshuMatrix]

Summary

• What changed:

  • Added a new background task recalculateAttachmentDecay that scans all attachment decay entries in ScyllaDB/Cassandra and retroactively applies the latest instance retention rules (or clears expiries if disabled).
  • Integrated the background task to run asynchronously whenever the media configuration is updated in InstanceConfigAdminController.
  • Updated the NATS/Rust microservice message mapping (fluxer_messages) to respect whether attachment decay is currently enabled when constructing message responses.

• Why it is correct:

  • Previously, media expiry was calculated statically only at the time of upload, meaning admin panel changes did not affect pre-existing attachments. The new background task ensures that settings are retroactively applied to all historical attachments.

• Risk:

  • Low. The background task queries ScyllaDB/Cassandra using pagination (100 records per page) to prevent query load issues.

Verification

• Tests run:

  • Created a new integration test suite AttachmentDecayRetroactive.test.ts verifying:
    • Scenario 1: Disabling decay retroactively clears expiries.
    • Scenario 2: Enabling decay retroactively applies the lifetime rules to pre-existing files.
  • Ran the test suite via Vitest:

    npx vitest run src/api/channel/tests/AttachmentDecayRetroactive.test.ts

    (Result: Both scenarios passed successfully).

• Manual checks:

  • Ran pnpm --filter fluxer_api typecheck and Biome format/lint checks successfully.

• Screenshots or recordings:

  • None.

Checklist

• I understand every change in this PR.
• I can explain what it does and why it is correct.
• I disclosed any LLM coding help below.

LLM Disclosure

• Only tests were generated with AI.

[jreoka]

Yes this is an awesome feature for selfhosters!