fidelity: push per-fund manual snapshot instead of gains-offset DEPOSIT

PlanViewer's DisplayValuation.action JSON already gives us current
fund units + unit price; we were parsing it and throwing it away,
emitting only a single 'unrealised-gains-offset' DEPOSIT to make
Wealthfolio's totals match the dashboard. That hack double-counted
the gain as a cash contribution, hiding £35k of pension growth from
every contribution/growth/ROI panel.

New flow:
- FidelityPlanViewerProvider exposes last_holdings + last_total_contribution
  after fetch() drains.
- fidelity-ingest CLI converts to a ManualSnapshotPayload (cost basis
  allocated proportionally by current fund value share) and posts to
  WF /api/v1/snapshots/import. WF auto-creates unknown fund symbols
  with kind=INVESTMENT, quoteMode=MANUAL, quoteCcy=GBP.
- The gains-offset emission is removed entirely. Historical offset
  rows already in WF are corrected at the dashboard layer by the
  dav_corrected view shipped in infra@2841347e.

WealthfolioSink gains push_manual_snapshots() + ManualSnapshotPayload /
SnapshotPosition wire types. 11 sink tests (3 new) + 9 fidelity
provider tests (2 changed, 1 new) all green; mypy + ruff clean.

tests/sinks/test_wealthfolio.py

@@ -1,7 +1,7 @@
 from __future__ import annotations
 
 import json
-from datetime import UTC, datetime
+from datetime import UTC, date, datetime
 from decimal import Decimal
 from pathlib import Path
 from typing import Any
@@ -12,6 +12,9 @@ import pytest
 from broker_sync.models import Account, AccountType, Activity, ActivityType
 from broker_sync.sinks.wealthfolio import (
     ImportValidationError,
+    ManualSnapshotPayload,
+    SnapshotPosition,
+    WealthfolioError,
     WealthfolioSink,
     WealthfolioUnauthorizedError,
 )
@@ -274,3 +277,99 @@ async def test_import_halts_on_validation_failure(tmp_path: Path) -> None:
     with pytest.raises(ImportValidationError, match="unknown symbol"):
         await sink.import_activities([_buy()])
     assert calls == ["/api/v1/activities/import/check"]  # real import never hit
+
+
+# -- Manual snapshot import (Fidelity path) --
+
+
+@pytest.mark.asyncio
+async def test_push_manual_snapshots_serialises_decimals_and_calls_endpoint(
+    tmp_path: Path,
+) -> None:
+    sp = tmp_path / "s.json"
+    sp.write_text(json.dumps({"cookies": {"wf_token": "fresh"}}))
+
+    seen: dict[str, Any] = {}
+
+    async def handler(req: httpx.Request) -> httpx.Response:
+        if req.url.path == "/api/v1/snapshots/import":
+            seen["body"] = json.loads(req.content)
+            return httpx.Response(
+                200,
+                json={"snapshotsImported": 1, "snapshotsFailed": 0, "errors": []},
+            )
+        return httpx.Response(404)
+
+    sink = _client(httpx.MockTransport(handler), sp)
+    snapshot = ManualSnapshotPayload(
+        date=date(2026, 5, 16),
+        currency="GBP",
+        positions=[
+            SnapshotPosition(
+                symbol="KDOA",
+                quantity=Decimal("4200.5"),
+                average_cost=Decimal("24.29"),
+                total_cost_basis=Decimal("102004.15"),
+                currency="GBP",
+            ),
+        ],
+        cash_balances={"GBP": Decimal(0)},
+    )
+    result = await sink.push_manual_snapshots(
+        account_id="a7d6208d-2bd6-4f85-bf54-b77984c78234",
+        snapshots=[snapshot],
+    )
+    assert result["snapshotsImported"] == 1
+    # Wire format: numeric fields are STRINGS (Decimal.__format__('f'))
+    body = seen["body"]
+    assert body["accountId"] == "a7d6208d-2bd6-4f85-bf54-b77984c78234"
+    pos = body["snapshots"][0]["positions"][0]
+    assert pos == {
+        "symbol": "KDOA",
+        "quantity": "4200.5",
+        "averageCost": "24.29",
+        "totalCostBasis": "102004.15",
+        "currency": "GBP",
+    }
+    assert body["snapshots"][0]["cashBalances"] == {"GBP": "0"}
+
+
+@pytest.mark.asyncio
+async def test_push_manual_snapshots_raises_on_partial_failure(
+    tmp_path: Path,
+) -> None:
+    sp = tmp_path / "s.json"
+    sp.write_text(json.dumps({"cookies": {"wf_token": "fresh"}}))
+
+    async def handler(req: httpx.Request) -> httpx.Response:
+        return httpx.Response(
+            200,
+            json={
+                "snapshotsImported": 0,
+                "snapshotsFailed": 1,
+                "errors": [{"row": 0, "msg": "bad symbol"}],
+            },
+        )
+
+    sink = _client(httpx.MockTransport(handler), sp)
+    snapshot = ManualSnapshotPayload(
+        date=date(2026, 5, 16), currency="GBP",
+        positions=[], cash_balances={},
+    )
+    with pytest.raises(WealthfolioError, match="bad symbol"):
+        await sink.push_manual_snapshots(account_id="acct", snapshots=[snapshot])
+
+
+@pytest.mark.asyncio
+async def test_push_manual_snapshots_short_circuits_on_empty(
+    tmp_path: Path,
+) -> None:
+    sp = tmp_path / "s.json"
+    sp.write_text(json.dumps({"cookies": {"wf_token": "fresh"}}))
+
+    async def handler(req: httpx.Request) -> httpx.Response:
+        raise AssertionError(f"unexpected request: {req.method} {req.url.path}")
+
+    sink = _client(httpx.MockTransport(handler), sp)
+    result = await sink.push_manual_snapshots(account_id="acct", snapshots=[])
+    assert result["snapshotsImported"] == 0