Add backlinks for document mentions

[priethor]

What

Follow-up to #376, closes #160.

Adds backlinks for document mentions. When a document mentions the document you are viewing, Cortext shows that source and lets you jump back to it.

In the document inspector, backlinks appear as a collapsible panel. In the side detail view, they sit behind a small toolbar button so they do not take over the editor body. Open backlink views also refresh after content is saved, and the side popover checks again when opened.

How

• Keeping a private mention index updated when documents are saved or deleted.
• Counting repeated mentions from the same source, while still showing that source only once.
• Adding a backfill command for existing content.
• Hiding backlink sources that the current viewer cannot read.

Testing Instructions

1. Open a document that is mentioned by another document.
2. Confirm backlinks appear on the mentioned document.
3. Click a backlink and confirm that Cortext opens the source document.
4. Mention the same target twice from one source, save, and confirm the count updates without duplicating the source.
5. Remove the mention, save, and confirm the backlink disappears.
6. Open the side detail backlink popover and confirm it shows the current count.
7. Run the backfill command on existing content and confirm backlinks still appear.

Screenshots