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.

broker_sync/sinks/wealthfolio.py

@@ -2,7 +2,9 @@ from __future__ import annotations
 
 import json
 from collections.abc import Iterable
-from datetime import UTC
+from dataclasses import dataclass
+from datetime import UTC, date
+from decimal import Decimal
 from pathlib import Path
 from typing import Any
 
@@ -14,6 +16,7 @@ _LOGIN_PATH = "/api/v1/auth/login"
 _ACCOUNTS_PATH = "/api/v1/accounts"
 _IMPORT_CHECK = "/api/v1/activities/import/check"
 _IMPORT_REAL = "/api/v1/activities/import"
+_SNAPSHOTS_IMPORT = "/api/v1/snapshots/import"
 
 
 class WealthfolioError(Exception):
@@ -262,3 +265,83 @@ class WealthfolioSink:
                 f"First warning: {first_warn}")
         assert isinstance(got, list)
         return [r for r in got if isinstance(r, dict)]
+
+    # -- manual holdings snapshots --
+
+    async def push_manual_snapshots(
+        self,
+        account_id: str,
+        snapshots: list[ManualSnapshotPayload],
+    ) -> dict[str, Any]:
+        """Push manual holdings snapshots to /api/v1/snapshots/import.
+
+        Each snapshot carries a date + per-fund positions + cash balances.
+        Wealthfolio auto-creates any unknown asset symbol with
+        ``kind=INVESTMENT, quoteMode=MANUAL, quoteCcy=<currency>`` and uses
+        the snapshot to derive holdings + valuation for that date — bypassing
+        the activity-ledger derivation entirely for the targeted day.
+
+        Used by the Fidelity provider since PlanViewer exposes current
+        fund units + price but no per-trade history. Re-imports for the
+        same (account, date) overwrite in place.
+        """
+        if not snapshots:
+            return {"snapshotsImported": 0, "snapshotsFailed": 0, "errors": []}
+        body = {
+            "accountId": account_id,
+            "snapshots": [_snapshot_to_payload(s) for s in snapshots],
+        }
+        resp = await self._request("POST", _SNAPSHOTS_IMPORT, json=body)
+        if resp.status_code >= 400:
+            try:
+                payload = resp.json()
+            except Exception:
+                payload = {"raw": resp.text}
+            raise WealthfolioError(
+                f"Wealthfolio /snapshots/import rejected: {payload}")
+        result = resp.json()
+        assert isinstance(result, dict)
+        failed = int(result.get("snapshotsFailed", 0))
+        if failed > 0:
+            raise WealthfolioError(
+                f"Wealthfolio /snapshots/import: {failed} snapshot(s) failed; "
+                f"errors={result.get('errors')}")
+        return result
+
+
+@dataclass(frozen=True)
+class SnapshotPosition:
+    """A per-fund position row in a Wealthfolio manual snapshot."""
+    symbol: str
+    quantity: Decimal
+    average_cost: Decimal
+    total_cost_basis: Decimal
+    currency: str
+
+
+@dataclass(frozen=True)
+class ManualSnapshotPayload:
+    """Sink-facing snapshot row. Mirrors the JSON shape WF expects."""
+    date: date
+    currency: str
+    positions: list[SnapshotPosition]
+    cash_balances: dict[str, Decimal]
+
+
+def _snapshot_to_payload(s: ManualSnapshotPayload) -> dict[str, Any]:
+    """Serialise a ManualSnapshotPayload into WF's import wire format."""
+    return {
+        "date": s.date.isoformat(),
+        "currency": s.currency,
+        "positions": [
+            {
+                "symbol": p.symbol,
+                "quantity": format(p.quantity, "f"),
+                "averageCost": format(p.average_cost, "f"),
+                "totalCostBasis": format(p.total_cost_basis, "f"),
+                "currency": p.currency,
+            }
+            for p in s.positions
+        ],
+        "cashBalances": {k: format(v, "f") for k, v in s.cash_balances.items()},
+    }