test(04-07): add failing sharee-403 tests for T-04-08 owner-only isShared guard

- T-04-08 test 1: sharee PATCH { isShared: false } must get 403 and
  list_shares row unchanged (currently 200 + shares wiped — bug)
- T-04-08 test 2: sharee PATCH { isShared: true } must get 403 and
  no new shares inserted (currently 200 + shares fan-out — bug)
- Both tests fail now; GREEN once owner-only guard added to lists.ts

apps/api/tests/routes/lists.test.ts

@@ -448,6 +448,56 @@ describe('PATCH /api/lists/:id (authorized update)', () => {
     const body = await res.json() as { name: string }
     expect(body.name).toBe('Sharee Renamed')
   })
+
+  it('T-04-08: sharee sending { isShared: false } gets 403 and list_shares is unchanged', async () => {
+    const ownerId = await seedUser('t04-08-owner')
+    const shareeId = await seedUser('t04-08-sharee')
+    const listId = await seedList(ownerId, 'T04-08 Shared List', true)
+    await shareList(listId, shareeId)
+
+    // Act as sharee
+    currentDevUserId = shareeId
+    const app = await getApp()
+
+    // Sharee tries to set isShared: false — must be blocked
+    const res = await app.request(jsonRequest('PATCH', `/api/lists/${listId}`, { isShared: false }))
+    expect(res.status).toBe(403)
+    const body = await res.json() as { error: string }
+    // Response should mention owner/sharing
+    expect(body.error).toMatch(/owner|shar/i)
+
+    // Prove list_shares is untouched — sharee row still exists
+    const { and, eq } = await import('drizzle-orm')
+    const shares = await db.select().from(listShares).where(
+      and(eq(listShares.listId, listId), eq(listShares.userId, shareeId)),
+    )
+    expect(shares.length).toBe(1)
+  })
+
+  it('T-04-08: sharee sending { isShared: true } gets 403 and no new shares are inserted', async () => {
+    const ownerId = await seedUser('t04-08-true-owner')
+    const shareeId = await seedUser('t04-08-true-sharee')
+    // Private list (isShared: false) but sharee already has explicit access
+    const listId = await seedList(ownerId, 'T04-08 Private List', false)
+    await shareList(listId, shareeId)
+
+    // Act as sharee
+    currentDevUserId = shareeId
+    const app = await getApp()
+
+    // Record share count before the attempted mutation
+    const { eq } = await import('drizzle-orm')
+    const beforeShares = await db.select().from(listShares).where(eq(listShares.listId, listId))
+    const beforeCount = beforeShares.length
+
+    // Sharee tries to set isShared: true — must be blocked
+    const res = await app.request(jsonRequest('PATCH', `/api/lists/${listId}`, { isShared: true }))
+    expect(res.status).toBe(403)
+
+    // Prove no new shares were inserted
+    const afterShares = await db.select().from(listShares).where(eq(listShares.listId, listId))
+    expect(afterShares.length).toBe(beforeCount)
+  })
 })
 
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