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/objects/12-mlag.yml

@@ -132,3 +132,49 @@ spec:
       heartbeat_peer_ip: "172.16.0.31"
       local_interface: ["leaf8", "Vlan4090"]
       peer_link: ["leaf8", "Port-Channel999"]
+---
+apiVersion: infrahub.app/v1
+kind: Object
+spec:
+  kind: InfraMlagInterface
+  data:
+    # MLAG interface 1 on leaf1/leaf2 pair (Port-Channel1 → host1)
+    - local_identifier: mlag1__leaf1-leaf2
+      mlag_id: 1
+      description: "MLAG interface 1 for leaf1/leaf2 pair (host1)"
+      lacp_fallback_timeout: 5
+      lacp_fallback_individual: true
+      mlag_domain: ["leafs-1-2"]
+      lag_interfaces:
+        - ["leaf1", "Port-Channel1"]
+        - ["leaf2", "Port-Channel1"]
+    # MLAG interface 1 on leaf3/leaf4 pair (Port-Channel1 → host2)
+    - local_identifier: mlag1__leaf3-leaf4
+      mlag_id: 1
+      description: "MLAG interface 1 for leaf3/leaf4 pair (host2)"
+      lacp_fallback_timeout: 5
+      lacp_fallback_individual: true
+      mlag_domain: ["leafs-3-4"]
+      lag_interfaces:
+        - ["leaf3", "Port-Channel1"]
+        - ["leaf4", "Port-Channel1"]
+    # MLAG interface 1 on leaf5/leaf6 pair (Port-Channel1 → host3)
+    - local_identifier: mlag1__leaf5-leaf6
+      mlag_id: 1
+      description: "MLAG interface 1 for leaf5/leaf6 pair (host3)"
+      lacp_fallback_timeout: 5
+      lacp_fallback_individual: true
+      mlag_domain: ["leafs-5-6"]
+      lag_interfaces:
+        - ["leaf5", "Port-Channel1"]
+        - ["leaf6", "Port-Channel1"]
+    # MLAG interface 1 on leaf7/leaf8 pair (Port-Channel1 → host4)
+    - local_identifier: mlag1__leaf7-leaf8
+      mlag_id: 1
+      description: "MLAG interface 1 for leaf7/leaf8 pair (host4)"
+      lacp_fallback_timeout: 5
+      lacp_fallback_individual: true
+      mlag_domain: ["leafs-7-8"]
+      lag_interfaces:
+        - ["leaf7", "Port-Channel1"]
+        - ["leaf8", "Port-Channel1"]