feat(restore): operator-configurable wallet restore policy

How far back a wallet may ask THIS instance to scan on import is an operator
decision — the backfill cost lands on its LWS/node. Add WALLET_RESTORE_POLICY
(create-only | bounded | unlimited, default bounded) + WALLET_MAX_RESTORE_DEPTH_DAYS,
advertise it via /capabilities so the wallet adapts its import UX, and enforce
it at the scan path (xmr/wow register, grin scan): a too-deep start_height is a
ValidationError naming the bound, never the caller's value. Create needs no
check (no tip lookup). Self-sovereignty stays intact at the ecosystem level —
a deeper restore means self-hosting or a looser operator. Fail-closed config
validation; a unit test per policy; OpenAPI drift clean.

openapi.json

@@ -1891,7 +1891,8 @@
           "version",
           "contract_version",
           "chains",
-          "features"
+          "features",
+          "restore"
         ],
         "properties": {
           "chains": {
@@ -1906,6 +1907,10 @@
           "features": {
             "$ref": "#/components/schemas/FeatureCapabilities"
           },
+          "restore": {
+            "$ref": "#/components/schemas/RestoreCapability",
+            "description": "Wallet restore (import) policy for this instance."
+          },
           "version": {
             "type": "string",
             "description": "Backend version (Cargo package version)."
@@ -2972,6 +2977,27 @@
           }
         }
       },
+      "RestoreCapability": {
+        "type": "object",
+        "description": "This instance's wallet-restore (import) policy. The wallet uses it to adapt\nits import UX — hide/grey the restore-height field under `create-only`, warn\nwhen a chosen date exceeds the bound. `max_depth_days` is present only for\nthe `bounded` policy.",
+        "required": [
+          "policy"
+        ],
+        "properties": {
+          "max_depth_days": {
+            "type": [
+              "integer",
+              "null"
+            ],
+            "format": "int32",
+            "minimum": 0
+          },
+          "policy": {
+            "type": "string",
+            "description": "`create-only` | `bounded` | `unlimited`."
+          }
+        }
+      },
       "SetUsernameRequest": {
         "type": "object",
         "required": [