Link imported attachments to their source email (cross-navigation + hierarchical export)

When you import an email attachment into a project as a document (see the “Import common document attachments as PDFs” work), the imported PDF and the email it came from should stay linked and easy to move between — both in the app and in the export.

1. Cross-navigation in the app

• From the imported document's PDF view, a link back to the source email it was extracted from.
• From the email, a link out to its imported attachment document(s).
• Make jumping between the two effortless — you're often redacting the email and its attachment as one piece of work.

2. Hierarchical export

• On export, the imported attachment PDF should sit “underneath” its parent email — a parent/child hierarchy rather than a flat list, so the relationship is obvious when someone reviews the exported bundle.
• The structure should make clear “this PDF came from that email”.

3. Linked attachment inside the exported PDF

• In the exported email PDF, embed a link / attached document pointing to the imported attachment PDF, so a reviewer can jump straight to it from within the email export.

Notes / scope:

• The data link already exists — imported documents store a source_email_id back to their email — so this is mostly UI navigation + export-structure work, not new plumbing.
• Depends on the convert-standard-docs import feature; logged separately to keep that one focused.
• Export piece needs design: how the hierarchy is represented (folder nesting in the ZIP, ordering in a combined PDF) and how the in-PDF link/attachment is generated (pdf-lib link annotation or embedded file).