[PM-41866] [server] add event logging for attachment downloads

[vlad-trofimov]

🎟️ Tracking

This is related to https://bitwarden.atlassian.net/browse/PFI-824

📔 Objective

Attachment interactions on vault items can be a critical action in regulated environments where organizations need a full audit trail of who accessed sensitive files and when. Today, Cipher_AttachmentCreated (1103) and Cipher_AttachmentDeleted (1104) are logged, but downloads are not. This leaves a gap in the event history for compliance reviews. This PR introduces a new event, Cipher_AttachmentDownloaded (1133), emitted server-side from CipherService.GetAttachmentDownloadDataAsync whenever a user requests the signed download URL for an attachment.
Because the server issues the URL, the event cannot be bypassed by a modified client - distinguishing it from client-reported Cipher_Client* events. Attachment metadata already flows on sync, so this signal is limited to genuine user-initiated download actions and won't inflate the event table with sync noise.

📸 Screenshots

[bitwarden-bot]

Thank you for your contribution! We've added this to our internal tracking system for review.
ID: PM-41866
Link: https://bitwarden.atlassian.net/browse/PM-41866

Details on our contribution process can be found here: https://contributing.bitwarden.com/contributing/pull-requests/community-pr-process.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 63.65%. Comparing base (6244dfc) to head (98e284e).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8190      +/-   ##
==========================================
- Coverage   63.89%   63.65%   -0.25%     
==========================================
  Files        2359     2366       +7     
  Lines      102319   102728     +409     
  Branches     9240     9320      +80     
==========================================
+ Hits        65378    65388      +10     
- Misses      34714    35113     +399     
  Partials     2227     2227              

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