[Feature]: Secure Send expiring external share links from file and chat

[DenizAltunkapan]

Problem

Vault Web can share files internally between accounts, but there is no way to send a file to someone who does not have an account. Users regularly need to hand a file to family or a friend without giving them access to the cloud or a folder. Today that is not possible without an internal account.

This is deliberately different from internal user-to-user sharing (#27): that grants another Vault Web user access to an item. Secure Send is about generating a self-contained external link that expires, with no account and no folder access on the other side.

Example: a user wants to send one document to a relative. They generate a link that works for 48 hours, optionally behind a password, and stops working afterwards. The relative never sees anything else in the cloud.

Proposed solution

Add Secure Send: an expiring, optionally password-protected external link to a single file, creatable from the file context and from a chat.

Scope:

• generate a share link for a single file, with an expiry time
• optional password protection on the link
• the link exposes only that one file, with no listing of or access to anything else in the user root
• create a Secure Send link from the file viewer sidebar ([Feature]: Workspace file viewer for images, video, audio, and PDF with sidebar actions #98) and share it directly into a chat (this is the Cloud plus Chat connection that makes it distinctive)
• links can be listed and revoked before they expire
• expired or revoked links return a clean error, not the file
• reuse the existing path validation and user boundary

Notes:

• Explicitly not a replacement for internal sharing ([Feature]: Implement secure file and folder sharing between users #27). Both can coexist: [Feature]: Implement secure file and folder sharing between users #27 for Vault Web users, Secure Send for outside recipients.
• Consider rate limiting link creation, consistent with the other Cloud rate limits.

Acceptance criteria

• A user can create an expiring external link to a single file.
• Links can optionally require a password.
• A link exposes only the target file and never lists or serves anything else in the user root.
• Links can be listed and revoked, and expired or revoked links return a clean error.
• A Secure Send link can be created from the file view and shared into a chat.
• The feature is clearly separate from internal user-to-user sharing ([Feature]: Implement secure file and folder sharing between users #27).
• Tests cover expiry, password protection, revocation, and that no other files are reachable.

Related: distinct from internal sharing (#27); created from the file viewer sidebar (#98).