fix(authz): scope import, approvals, activity, and heartbeat routes (#3315)

## Thinking Path

> - Paperclip orchestrates AI agents and company-scoped control-plane
actions for zero-human companies.
> - This change touches the server authz boundary around company
portability, approvals, activity, and heartbeat-run operations.
> - The vulnerability was that board-authenticated callers could cross
company boundaries or create new companies through import paths without
the same authorization checks enforced elsewhere.
> - Once that gap existed, an attacker could chain it into higher-impact
behavior through agent execution paths.
> - The fix needed to harden every confirmed authorization gap in the
reported chain, not just the first route that exposed it.
> - This pull request adds the missing instance-admin and company-access
checks and adds regression tests for each affected route.
> - The benefit is that cross-company actions and new-company import
flows now follow the same control-plane authorization rules as the rest
of the product.

## What Changed

- Required instance-admin access for `new_company` import preview/apply
flows in `server/src/routes/companies.ts`.
- Required company access before approval decision routes in
`server/src/routes/approvals.ts`.
- Required company access for activity creation and heartbeat-run issue
listing in `server/src/routes/activity.ts`.
- Required company access before heartbeat cancellation in
`server/src/routes/agents.ts`.
- Added regression coverage in the corresponding server route tests.

## Verification

- `pnpm --filter @paperclipai/server exec vitest run
src/__tests__/company-portability-routes.test.ts
src/__tests__/approval-routes-idempotency.test.ts
src/__tests__/activity-routes.test.ts
src/__tests__/agent-permissions-routes.test.ts`
- `pnpm --filter @paperclipai/server typecheck`
- Prior verification on the original security patch branch also included
`pnpm build`.

## Risks

- Low code risk: the change is narrow and only adds missing
authorization gates to existing routes.
- Operational risk: the advisory is already public, so this PR should be
merged quickly to minimize the public unpatched window.
- Residual product risk remains around open signup / bootstrap defaults,
which was intentionally left out of this patch because the current
first-user onboarding flow depends on it.

## Model Used

- OpenAI GPT-5 Codex coding agent with tool use and local code execution
in the Codex CLI environment.

## Checklist

- [x] I have included a thinking path that traces from project context
to this change
- [x] I have specified the model used (with version and capability
details)
- [x] I have run tests locally and they pass
- [x] I have added or updated tests where applicable
- [ ] If this change affects the UI, I have included before/after
screenshots
- [ ] I have updated relevant documentation to reflect my changes
- [x] I have considered and documented any risks above
- [x] I will address all Greptile and reviewer comments before
requesting merge

Co-authored-by: Forgotten <forgottenrunes@protonmail.com>

server/src/__tests__/agent-permissions-routes.test.ts

@@ -59,6 +59,8 @@ const mockBudgetService = vi.hoisted(() => ({
 const mockHeartbeatService = vi.hoisted(() => ({
   listTaskSessions: vi.fn(),
   resetRuntimeSession: vi.fn(),
+  getRun: vi.fn(),
+  cancelRun: vi.fn(),
 }));
 
 const mockIssueApprovalService = vi.hoisted(() => ({
@@ -397,4 +399,26 @@ describe("agent permission routes", () => {
       },
     ]);
   });
+
+  it("rejects heartbeat cancellation outside the caller company scope", async () => {
+    mockHeartbeatService.getRun.mockResolvedValue({
+      id: "run-1",
+      companyId: "33333333-3333-4333-8333-333333333333",
+      agentId,
+      status: "running",
+    });
+
+    const app = await createApp({
+      type: "board",
+      userId: "board-user",
+      source: "session",
+      isInstanceAdmin: false,
+      companyIds: [companyId],
+    });
+
+    const res = await request(app).post("/api/heartbeat-runs/run-1/cancel").send({});
+
+    expect(res.status).toBe(403);
+    expect(mockHeartbeatService.cancelRun).not.toHaveBeenCalled();
+  });
 });