Refactor: Remove properties.root_id from mentions relationships

[malibio]

Summary

The mentions relationship currently stores properties.root_id on the relationship edge to speed up backlinks queries. This is denormalized/cached data that can become stale and adds complexity. We should compute roots dynamically using recursive graph traversal.

Current Behavior

When creating a mention:

1. create_mention() computes the root of the source node via get_root_id() traversal
2. Stores it as properties.root_id on the relationship
3. get_mentioning_containers() reads properties.root_id to find backlink sources

Proposed Change

Phase 1: Implement dynamic root lookup

Refactor get_mentioning_containers() to compute roots dynamically:

1. Get mentions where out = target_node
2. For each mention's in (source), recursively traverse UP via has_child relationships to find the root
3. Use the same recursive traversal pattern used for parent-child fetching (but going UP instead of DOWN)
4. Return unique root nodes

Phase 2: Update frontend integration

Ensure the backlinks panel uses the refactored get_mentioning_containers():

• Backlinks query runs in parallel with descendant loading (not awaiting each other)
• Both update different parts of the UI independently

Phase 3: Remove properties.root_id from mention creation

1. Remove root_id parameter from create_mention() in surreal_store.rs
2. Remove root_id computation and storage in node_service.rs
3. Simplify the mention relationship to just in (source) → out (target)

Phase 4: Migration cleanup (post-release consideration)

For existing installations, optionally clean up old data:

UPDATE relationship SET properties.root_id = NONE WHERE relationship_type = 'mentions';

Or leave it - the field becomes ignored dead data (no schema change needed since properties is flexible).

Benefits

• Always accurate (reflects current hierarchy)
• Simpler data model
• No stale data if nodes move
• Cleaner create_mention logic
• Consistent pattern with other recursive traversals

Files to Modify

• packages/core/src/db/surreal_store.rs - get_mentioning_containers(), create_mention()
• packages/core/src/services/node_service.rs - create_mention()

[malibio]

Additional Context: Recursive Query Approach

The refactored get_mentioning_containers() should use the same recursive traversal pattern used for parent-child fetching.

The query would:

1. Find all mentions where out = target_node
2. For each mention's in (source), recursively traverse UP via has_child relationships to find the root
3. Return unique root nodes

This mirrors how we recursively traverse DOWN for children, but goes UP to find ancestors. Both queries (loading descendants and finding backlink roots) run in parallel when a root node is loaded - they update different parts of the UI independently.

[malibio]

Test baseline: Frontend: 3478 passed (102 test files), Rust: 732 passed