Feat: Create Asset Link

[txalkan]

• Adds POST /assetlink/create.
• Persists a local AssetLink record per issued asset in KV storage (issue_asset_nia now writes an issuer record with linked_asset_id = None, signature = None, created_at = None).
• asset_link_create then upgrades the asset_id record to point at linked_asset_id, adds the host signature, and sets created_at.

Validation rules:

• Both assets must exist locally and be issued by the same node pubkey.
• asset_id and linked_asset_id must differ.
• Repeating the same link is idempotent.
• Linking the same asset to a different target is rejected.