feat(server-mode): share a workspace with other users (sharing phase 1)

A workspace's owner (or an admin) shares it from its ⋯ menu → Share…
(lane_shares, GET/PUT /api/lanes/:id/shares, audited). The users it is shared
with see it, its sessions, Changes, history and live updates, join and type
into its open terminals, commit and push — every check through scope.js
(canSeeLane, sessionWhere, laneWhere, broadcastAudience viewers). Rename,
reset, remove, delete and sharing stay the owner's or an admin's
(403 ENOTOWNER), and a collaborator starts no AI session there yet
(403 ECOLLABSTART; a plain terminal tab is fine). Budgets and Today stay on
owned rows only (ownedOnlySessionWhere). Taking a share back sends lane_access
to those users and their terminals close at the next attach re-check.

Under S3 a collaborator works as the owner's Linux account; the Share dialog
says so plainly (docs/plans/workspace-sharing/, R1 option a).

server/openapi-extra/lanes-more.js

@@ -701,6 +701,43 @@ const paths = {
       },
     },
   },
+  "/api/lanes/{id}/shares": {
+    get: {
+      tags: ["Lanes"],
+      summary: "The users a workspace is shared with",
+      description:
+        "Server mode (local mode: `{shares: [], canManage: false}`). `{shares:[{id, username, name, sharedAt}], canManage}` — names only; `canManage` is whether this caller (the owner or an admin) may change the list. A user it is shared with may read it too.",
+      operationId: "getLaneShares",
+      parameters: [idParam],
+      responses: { 200: { description: "`{shares, canManage}`.", ...json(obj) }, 404: notFound },
+    },
+    put: {
+      tags: ["Lanes"],
+      summary: "Set the users a workspace is shared with",
+      description:
+        "`{userIds:[…]}` is the WHOLE list (≤ 200 enabled users, never the owner). The owner or an admin only (`403 ENOTOWNER`); same-origin; each user added or taken off is written to the audit log. A user it is shared with sees it and works in it like its owner — under S3 as the owner's Linux account — except rename, reset, remove, delete and sharing (`ENOTOWNER`), and starts no AI session in it yet (`ECOLLABSTART`). Users taken off get `lane_access` and lose access at once.",
+      operationId: "setLaneShares",
+      parameters: [idParam],
+      requestBody: {
+        required: true,
+        content: {
+          "application/json": {
+            schema: {
+              type: "object",
+              required: ["userIds"],
+              properties: { userIds: { type: "array", items: { type: "integer" } } },
+            },
+          },
+        },
+      },
+      responses: {
+        200: { description: "`{shares}`.", ...json(obj) },
+        400: { description: "EBADUSER or ELOCALMODE." },
+        403: { description: "ENOTOWNER." },
+        404: notFound,
+      },
+    },
+  },
   "/api/lanes/{id}/memory": {
     get: {
       tags: ["Lanes"],