feat(grin): proxy scans through grin-lws (default) with grin-wallet fallback

Add a GrinLwsClient (a faithful mirror of the XMR LwsClient) + GrinLwsConfig
(enabled when GRIN_LWS_URL is set). /wallet/grin/scan now defaults to grin-lws
when configured and returns derivation paths (key_id/n_child) + spendability,
letting the wallet skip its client-side output-identify search.

Money-safety: the restore-depth/PoW gate runs once and is terminal (never
masked by fallback); grin-lws is trusted only when provably synced
(scanned_height + margin >= tip), otherwise the scan falls back to the
authoritative grin-wallet scan_rewind_hash; a grin-lws transport error also
falls back; grin-wallet errors propagate as 503. No path ever synthesizes an
empty/zero balance.

Off unless GRIN_LWS_URL is configured (prod grin behavior unchanged until
pointed at a grin-lws instance). Capabilities advertise `grin_lws`.
openapi.json regenerated.

openapi.json

@@ -895,6 +895,47 @@
         }
       }
     },
+    "/wallet/grin/address/{addr}/user": {
+      "get": {
+        "tags": [
+          "grin"
+        ],
+        "summary": "Resolve a bare Grin address to its owning user's Nostr routing identity.",
+        "description": "The address→npub bridge (see [`GrinAddressUserResponse`]). JWT-gated: only\nauthenticated senders resolve routing. An address unknown to this backend\nreturns the constant-shape `registered: false` response — the caller then\nfalls back to manual clipboard or the same-instance relay.",
+        "operationId": "address_user",
+        "parameters": [
+          {
+            "name": "addr",
+            "in": "path",
+            "description": "Grin address (registered public key)",
+            "required": true,
+            "schema": {
+              "type": "string"
+            }
+          }
+        ],
+        "responses": {
+          "200": {
+            "description": "Address resolution result",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "$ref": "#/components/schemas/GrinAddressUserResponse"
+                }
+              }
+            }
+          },
+          "401": {
+            "description": "Missing or invalid token"
+          }
+        },
+        "security": [
+          {
+            "bearer_auth": []
+          }
+        ]
+      }
+    },
     "/wallet/grin/broadcast": {
       "post": {
         "tags": [
@@ -2467,6 +2508,7 @@
         "type": "object",
         "required": [
           "grin_relay",
+          "grin_lws",
           "prices",
           "nostr_identity",
           "nostr_native_auth",
@@ -2480,6 +2522,10 @@
             "type": "boolean",
             "description": "Public curated Nostr feed for this instance. See `feed`."
           },
+          "grin_lws": {
+            "type": "boolean",
+            "description": "Grin light-wallet-server (grin-lws) scan path is available — fast balance\nscans via grin-lws, with the authoritative grin-wallet scan as fallback."
+          },
           "grin_relay": {
             "type": "boolean",
             "description": "Grin async slatepack relay mailbox."
@@ -2612,6 +2658,33 @@
           }
         }
       },
+      "GrinAddressUserResponse": {
+        "type": "object",
+        "description": "Resolution of a bare Grin address to its owning user's routing identity.\n\nThe address→npub **bridge**: given a `grin1…` slatepack address, report\nwhether it belongs to a user registered on *this* backend and, if so, that\nuser's linked Nostr pubkey so a bare-address send can be upgraded to a\nfederated Nostr gift-wrap instead of the same-instance relay.\n\nNOTE (federation): this is a **same-instance convenience**. It can only\nresolve addresses whose owner registered their Grin key here (via `POST\n/keys`). It is *not* a federated directory — a wallet on another backend\nwon't be found. `registered: false` (a constant-shape miss, mirroring\n`GET /users/by-username`) means \"unknown to this instance\", at which point\nthe sender falls back to manual clipboard / the backend relay.",
+        "required": [
+          "registered"
+        ],
+        "properties": {
+          "npub": {
+            "type": [
+              "string",
+              "null"
+            ],
+            "description": "The owning user's linked Nostr pubkey (x-only hex), when they have one.\nThis is what the sender feeds the Nostr gift-wrap channel to route the\nslate over Nostr. `None` when the user registered no Nostr identity."
+          },
+          "registered": {
+            "type": "boolean",
+            "description": "Whether the address resolved to a user on this backend."
+          },
+          "user_id": {
+            "type": [
+              "string",
+              "null"
+            ],
+            "description": "The owning user's id (UUID), when `registered`."
+          }
+        }
+      },
       "GrinBroadcastRequest": {
         "type": "object",
         "description": "Broadcast a finalized, signed Grin transaction (built + signed by the wallet).",
@@ -2672,6 +2745,13 @@
           "is_coinbase": {
             "type": "boolean"
           },
+          "key_id": {
+            "type": [
+              "string",
+              "null"
+            ],
+            "description": "Recovered derivation key id — populated only on the grin-lws path."
+          },
           "lock_height": {
             "type": "integer",
             "format": "int64",
@@ -2682,6 +2762,22 @@
             "format": "int64",
             "minimum": 0
           },
+          "n_child": {
+            "type": [
+              "integer",
+              "null"
+            ],
+            "format": "int32",
+            "description": "Recovered child index — populated only on the grin-lws path.",
+            "minimum": 0
+          },
+          "spendable": {
+            "type": [
+              "boolean",
+              "null"
+            ],
+            "description": "Spendable at the current tip — populated only on the grin-lws path."
+          },
           "value": {
             "type": "integer",
             "format": "int64",
@@ -2729,6 +2825,15 @@
           "last_pmmr_index"
         ],
         "properties": {
+          "blockchain_height": {
+            "type": [
+              "integer",
+              "null"
+            ],
+            "format": "int64",
+            "description": "Chain tip observed by grin-lws at scan time — present only on the grin-lws\npath.",
+            "minimum": 0
+          },
           "last_pmmr_index": {
             "type": "integer",
             "format": "int64",
@@ -2741,6 +2846,15 @@
               "$ref": "#/components/schemas/GrinOutput"
             }
           },
+          "scanned_height": {
+            "type": [
+              "integer",
+              "null"
+            ],
+            "format": "int64",
+            "description": "How far grin-lws has scanned this account — present only on the grin-lws\npath (the authoritative grin-wallet path leaves it null).",
+            "minimum": 0
+          },
           "total_balance": {
             "type": "integer",
             "format": "int64",