Discovery: persist explicit network relationships and conflict links

[BadgerOps]

Follow-up to #187 and PR #188.

Summary

Add durable relationship records between discovered resources, managed pools, allocated blocks, managed network objects, and conflict evidence.

Why

PR #188 computes links and conflicts at read time. Durable relationship records would make soft links, matches, missing parents, duplicates, and imported relationships first-class and queryable.

Scope

• Add a relationship/link model for contains, matches, conflicts, missing_parent, candidate_import, imported_as, and duplicate_of.
• Store confidence, reason/evidence, resolution state, timestamps, and target resource references.
• Expose API endpoints or embed relationships in existing network/discovery responses.
• Ensure links remain separate from import state.

Acceptance Criteria

• Soft links and conflict links survive recomputation and process restart.
• Merged views can read durable relationships when present.
• Tests cover relationship creation, listing, filtering, and resolution-state updates.