pipeline: emit matching DEPOSIT/WITHDRAWAL for every BUY/SELL

## Context

The 2026-04-18 reconciliation ended with Wealthfolio's historical Net
Worth chart showing cliff-jumps on 5 dates — the single-day lump cash
offsets we'd posted to "zero out" phantom cash. An operational fix
replaced those 6 lumps with 231 per-BUY/SELL matched DEPOSIT/WITHDRAWAL
rows (see code-r9n note). That made the chart smooth — but only for
today's data. Any future broker-sync run would re-introduce phantom
cash because providers emit BUY/SELL only; nothing on the cash side.

This commit bakes the match into the pipeline so **future syncs
self-balance cash at import time** and the chart stays smooth.

## This change

- broker_sync/pipeline.py
  - New _matched_cash_flow(a): returns a DEPOSIT for a BUY (amount =
    qty * unit_price + fee) or a WITHDRAWAL for a SELL (amount =
    qty * unit_price - fee). Returns None for every other activity
    type — DEPOSIT/WITHDRAWAL/DIVIDEND/etc. already touch cash
    directly. The synthetic activity carries a deterministic
    external_id `cash-flow-match:<buy|sell>:<original external_id>`
    so SyncRecordStore dedup handles idempotency across runs.
  - New _with_cash_flow_match(a): expand helper — returns [a] or
    [a, match]. Pure, testable.
  - sync_provider_to_wealthfolio loops over the expansion, so each
    activity may now contribute up to two rows to the batch. `fetched`
    still counts provider-side activities only; `new_after_dedup` +
    `imported` + `failed` count expanded rows.
- tests/test_pipeline.py
  - Updated two existing pipeline integration tests to reflect the
    now-larger batch shape (3 BUYs become 6 rows after expansion).
  - 5 new unit tests for the helpers: BUY → DEPOSIT with fee,
    SELL → WITHDRAWAL net of fee, DEPOSIT/WITHDRAWAL/DIVIDEND pass
    through, zero-amount trades skipped, _with_cash_flow_match
    returns the right cardinality.

## What is NOT in this change

- Provider-level opt-out (e.g., Provider.emits_matching_cash_flow =
  True). No current provider emits real cash flows alongside trades
  (Trading212 only calls /orders, not /transactions), so the default
  "always match" is safe. If we ever wire a provider that pulls real
  bank-transfer dates, add the opt-out then.
- Retroactive cleanup of already-imported WF accounts — already done
  operationally today.

## Verification

### Automated

$ poetry run pytest tests/test_pipeline.py -v
7 passed in 0.40s

$ poetry run pytest -q
133 passed, 1 skipped in 8.58s

$ poetry run mypy broker_sync/pipeline.py tests/test_pipeline.py
Success: no issues found in 2 source files

$ poetry run ruff check broker_sync/pipeline.py tests/test_pipeline.py
All checks passed!

### Manual — next sync

Once this image ships and broker-sync-trading212 / broker-sync-imap /
broker-sync-fidelity run, confirm:
1. kubectl -n broker-sync logs job/<next-run> → fetched=N new=2N
   imported=2N failed=0 (doubled due to matches).
2. WF /api/v1/holdings?accountId=<uuid> → cash ≈ £0 for every currency
   after import.
3. Net Worth chart has no new cliff-jumps.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/test_pipeline.py

@@ -119,21 +119,31 @@ async def test_pipeline_skips_dedup_then_imports_new(tmp_path: Path) -> None:
     finally:
         await sink.close()
 
+    # 3 provider activities fetched, but pipeline expands each BUY into
+    # (BUY, matching DEPOSIT). "a" is already-seen → skipped; its match
+    # "cash-flow-match:buy:a" is NEW since it wasn't seeded.
     assert result.fetched == 3
-    assert result.new_after_dedup == 2
-    assert result.imported == 2
+    assert result.new_after_dedup == 5
+    assert result.imported == 5
     assert result.failed == 0
     assert len(posted_batches) == 1
     body = posted_batches[0]
-    # Only the new rows (b, c) — NOT the already-seen "a".
+    # Only the new rows (b, c + the 3 matches) — NOT the already-seen "a".
     assert "sync:fake:a" not in body
     assert "sync:fake:b" in body
     assert "sync:fake:c" in body
+    # Matching DEPOSITs rode along with their trade.
+    assert "cash-flow-match:buy:a" in body
+    assert "cash-flow-match:buy:b" in body
+    assert "cash-flow-match:buy:c" in body
 
-    # All three external_ids are now in dedup after the run.
+    # All six external_ids are now in dedup after the run.
     assert dedup.has_seen("fake", "fake-isa", "a")
     assert dedup.has_seen("fake", "fake-isa", "b")
     assert dedup.has_seen("fake", "fake-isa", "c")
+    assert dedup.has_seen("fake", "fake-isa", "cash-flow-match:buy:a")
+    assert dedup.has_seen("fake", "fake-isa", "cash-flow-match:buy:b")
+    assert dedup.has_seen("fake", "fake-isa", "cash-flow-match:buy:c")
 
 
 async def test_pipeline_records_failure_when_import_rejects(tmp_path: Path) -> None:
@@ -172,8 +182,86 @@ async def test_pipeline_records_failure_when_import_rejects(tmp_path: Path) -> N
     finally:
         await sink.close()
 
+    # Pipeline expands 1 BUY into (BUY, matching DEPOSIT). Both are in the
+    # batch that /import/check rejects, so both are counted as failed.
     assert result.fetched == 1
     assert result.imported == 0
-    assert result.failed == 1
-    # NOT recorded in dedup so the next run retries.
+    assert result.failed == 2
+    # NOT recorded in dedup so the next run retries both.
     assert not dedup.has_seen("fake", "fake-isa", "a")
+    assert not dedup.has_seen("fake", "fake-isa", "cash-flow-match:buy:a")
+
+
+# -- Cash-flow match helpers ---------------------------------------------
+from broker_sync.pipeline import _matched_cash_flow, _with_cash_flow_match  # noqa: E402
+
+
+def _make_activity(
+    activity_type: ActivityType,
+    *,
+    quantity: str | None = "1",
+    unit_price: str | None = "100",
+    fee: str = "0",
+    amount: str | None = None,
+    external_id: str = "x",
+) -> Activity:
+    return Activity(
+        external_id=external_id,
+        account_id="acct",
+        account_type=AccountType.ISA,
+        date=datetime(2026, 4, 1, tzinfo=UTC),
+        activity_type=activity_type,
+        currency="GBP",
+        quantity=Decimal(quantity) if quantity is not None else None,
+        unit_price=Decimal(unit_price) if unit_price is not None else None,
+        fee=Decimal(fee),
+        amount=Decimal(amount) if amount is not None else None,
+    )
+
+
+def test_matched_cash_flow_for_buy_is_deposit_with_total_cost() -> None:
+    buy = _make_activity(
+        ActivityType.BUY, quantity="10", unit_price="200.50", fee="1.25",
+        external_id="buy-1",
+    )
+    match = _matched_cash_flow(buy)
+    assert match is not None
+    assert match.activity_type is ActivityType.DEPOSIT
+    assert match.amount == Decimal("2006.25")  # 10*200.50 + 1.25
+    assert match.currency == "GBP"
+    assert match.account_id == buy.account_id
+    assert match.date == buy.date
+    assert match.external_id == "cash-flow-match:buy:buy-1"
+
+
+def test_matched_cash_flow_for_sell_is_withdrawal_net_of_fee() -> None:
+    sell = _make_activity(
+        ActivityType.SELL, quantity="5", unit_price="300", fee="2.50",
+        external_id="sell-7",
+    )
+    match = _matched_cash_flow(sell)
+    assert match is not None
+    assert match.activity_type is ActivityType.WITHDRAWAL
+    assert match.amount == Decimal("1497.50")  # 5*300 - 2.50
+    assert match.external_id == "cash-flow-match:sell:sell-7"
+
+
+def test_matched_cash_flow_none_for_deposit_withdrawal_dividend() -> None:
+    dep = _make_activity(ActivityType.DEPOSIT, quantity=None, unit_price=None, amount="100")
+    wit = _make_activity(ActivityType.WITHDRAWAL, quantity=None, unit_price=None, amount="50")
+    div = _make_activity(ActivityType.DIVIDEND, quantity=None, unit_price=None, amount="5")
+    assert _matched_cash_flow(dep) is None
+    assert _matched_cash_flow(wit) is None
+    assert _matched_cash_flow(div) is None
+
+
+def test_matched_cash_flow_skips_zero_amount_trades() -> None:
+    zero_buy = _make_activity(ActivityType.BUY, quantity="0", unit_price="100")
+    assert _matched_cash_flow(zero_buy) is None
+
+
+def test_with_cash_flow_match_returns_pair_for_buy_single_for_deposit() -> None:
+    buy = _make_activity(ActivityType.BUY, external_id="buy-2")
+    dep = _make_activity(ActivityType.DEPOSIT, quantity=None, unit_price=None, amount="500")
+    assert len(_with_cash_flow_match(buy)) == 2
+    assert len(_with_cash_flow_match(dep)) == 1