fix(l1): T8 review fixes — oauth status const + bulk-invite structured error

- oauth.py: use status.HTTP_402_PAYMENT_REQUIRED constant (was raw 402)
- accounts.py bulk-invite: catch HTTPException separately to preserve
  structured detail dict in failed-row error (was stringified repr,
  unparseable by clients)
- Add bulk-invite per-row 402 test verifying structured error preserved

T8 code review identified these as Important issues. Functional change is
the bulk-invite fix; clients can now parse seat-limit errors from bulk
responses. 13/13 seat-enforcement tests pass.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/tests/test_invite_seat_enforcement.py

@@ -175,6 +175,39 @@ async def test_invite_unlimited_seat_limit_always_succeeds(client: AsyncClient,
     assert resp.status_code == 201, resp.text
 
 
+@pytest.mark.asyncio
+async def test_bulk_invite_per_row_402_preserves_structured_detail(client: AsyncClient, test_db: AsyncSession):
+    """Bulk invite returns 200 overall; rows that hit the seat limit appear in the
+    `failed` list with structured detail (not a stringified repr)."""
+    owner = await _register(client, email="owner_bulk@example.com")
+    account_id = uuid.UUID(owner["account_id"])
+    headers = await _login(client, email="owner_bulk@example.com")
+
+    # seat_limit=1, already 1 engineer → next engineer invite fails
+    await _set_sub(test_db, account_id, seat_limit=1)
+    await _add_member(test_db, account_id, role="engineer")
+
+    resp = await client.post(
+        "/api/v1/accounts/me/invites/bulk",
+        json={"invites": [
+            {"email": "viewer-ok@example.com", "role": "viewer"},
+            {"email": "eng-blocked@example.com", "role": "engineer"},
+        ]},
+        headers=headers,
+    )
+    assert resp.status_code in (200, 201), resp.text
+    body = resp.json()
+    assert len(body["created"]) == 1
+    assert body["created"][0]["email"] == "viewer-ok@example.com"
+    assert len(body["failed"]) == 1
+    failed_row = body["failed"][0]
+    assert failed_row["email"] == "eng-blocked@example.com"
+    # Structured detail preserved (dict, not repr string)
+    assert isinstance(failed_row["error"], dict)
+    assert failed_row["error"]["code"] == "seat_limit_exceeded"
+    assert failed_row["error"]["role"] == "engineer"
+
+
 @pytest.mark.asyncio
 async def test_invite_grandfathered_account_blocks_new_invites(client: AsyncClient, test_db: AsyncSession):
     """Grandfathering: existing over-seated account keeps existing users but