fix: add human_friendly_id to 4 schemas for idempotent Repository sync (#29)

## Summary

- Add `local_identifier` attribute + `uniqueness_constraints` + `human_friendly_id` to **InfraVlanVniMapping**, **InfraEVPNInstance**, **InfraMlagInterface**, and **InfraUnderlayLink**, following the same pattern already used for `InfraBGPPeerGroup` and `InfraBGPSession`
- Populate `local_identifier` in all corresponding object YAML files with deterministic composite keys so Infrahub can upsert objects during Repository backend sync instead of creating duplicates

## Schema changes

| Schema file | Model | human_friendly_id |
|---|---|---|
| `infrahub/schemas/vlan_vxlan.yml` | `InfraVlanVniMapping` | `local_identifier__value` |
| `infrahub/schemas/vlan_vxlan.yml` | `InfraEVPNInstance` | `local_identifier__value` |
| `infrahub/schemas/mlag.yml` | `InfraMlagInterface` | `local_identifier__value` |
| `infrahub/schemas/extensions.yml` | `InfraUnderlayLink` | `local_identifier__value` |

## Object file changes

| Object file | Kind | Key format | Example |
|---|---|---|---|
| `infrahub/objects/06-vlans-vxlan.yml` | `InfraVlanVniMapping` | `{device}__vlan{vlan_id}__vni{vni_id}` | `leaf1__vlan40__vni110040` |
| `infrahub/objects/06-vlans-vxlan.yml` | `InfraEVPNInstance` | `{device}__vlan{vlan_id}` | `leaf1__vlan40` |
| `infrahub/objects/12-mlag.yml` | `InfraMlagInterface` | `mlag{id}__{device1}-{device2}` | `mlag1__leaf1-leaf2` |
| `infrahub/objects/14-fabric-links.yml` | `InfraUnderlayLink` | `{local_dev}-{local_intf}__{remote_dev}-{remote_intf}` | `spine1-eth1__leaf1-eth11` |

Note: `InfraMlagInterface` objects (4 entries, one per MLAG pair) and `InfraUnderlayLink` objects (16 entries, all spine↔leaf P2P links) are new additions to their respective files.

## Test plan

- [x] `infrahubctl schema load infrahub/schemas/*.yml` succeeds without errors
- [x] `infrahubctl object load infrahub/objects/06-vlans-vxlan.yml` succeeds
- [x] `infrahubctl object load infrahub/objects/12-mlag.yml` succeeds
- [x] `infrahubctl object load infrahub/objects/14-fabric-links.yml` succeeds
- [x] Re-running the same load is idempotent (no duplicate objects created)
- [x] Gitea Repository backend sync completes without import errors

infrahub/schemas/vlan_vxlan.yml

@@ -174,8 +174,15 @@ nodes:
     label: VLAN-VNI Mapping
     icon: mdi--swap-horizontal
     include_in_menu: false
+    uniqueness_constraints:
+      - ["local_identifier__value"]
+    human_friendly_id:
+      - local_identifier__value
     display_label: "{{ description__value }}"
     attributes:
+      - name: local_identifier
+        kind: Text
+        description: "Unique identifier combining device name, VLAN ID and VNI (e.g. leaf1__vlan40__vni110040)"
       - name: description
         kind: Text
         optional: true
@@ -202,8 +209,15 @@ nodes:
     label: EVPN Instance
     icon: mdi--cloud-sync
     include_in_menu: false
+    uniqueness_constraints:
+      - ["local_identifier__value"]
+    human_friendly_id:
+      - local_identifier__value
     display_label: "{{ route_distinguisher__value }}"
     attributes:
+      - name: local_identifier
+        kind: Text
+        description: "Unique identifier combining device name and VLAN ID (e.g. leaf1__vlan40)"
       - name: route_distinguisher
         kind: Text
         description: "Route Distinguisher (format: ASN:VNI or IP:VNI)"