fidelity: replace snapshot-push with delta gains-offset DEPOSITs

Per-fund snapshot import landed quantities but dropped cost basis +
needed a separate quote-push path we never identified. Snapshotting
also collided with WF's own TOTAL aggregation and ZEROED the Fidelity
cash balance.

Simpler plan: each monthly scrape emits a single DEPOSIT (or
WITHDRAWAL on a market drop) sized to the delta between the live
PlanViewer pot value and Wealthfolio's running total. dav_corrected
PG view continues to subtract these offsets from net_contribution so
the dashboard Growth/ROI math stays right.

- New gains_offset_delta_activity() — current_gain - prior_offset.
- New WealthfolioSink.cumulative_amount_with_notes_prefix() — sums
  the existing fidelity-planviewer:unrealised-gains-offset DEPOSITs
  in WF so we know what's already been emitted.
- CLI runs sync_provider_to_wealthfolio first (cash flows), then
  computes + emits the delta via import_activities.
- 4 new provider tests for the delta logic; full suite (144 + 1
  skipped) green; mypy + ruff clean.

The old fidelity_holdings_to_snapshot helper + push_manual_snapshots
sink method stay for future use but are no longer called.

broker_sync/sinks/wealthfolio.py

@@ -17,6 +17,7 @@ _ACCOUNTS_PATH = "/api/v1/accounts"
 _IMPORT_CHECK = "/api/v1/activities/import/check"
 _IMPORT_REAL = "/api/v1/activities/import"
 _SNAPSHOTS_IMPORT = "/api/v1/snapshots/import"
+_ACTIVITIES_SEARCH = "/api/v1/activities/search"
 
 
 class WealthfolioError(Exception):
@@ -266,6 +267,54 @@ class WealthfolioSink:
         assert isinstance(got, list)
         return [r for r in got if isinstance(r, dict)]
 
+    # -- activity lookups --
+
+    async def cumulative_amount_with_notes_prefix(
+        self,
+        account_id: str,
+        notes_prefix: str,
+    ) -> Decimal:
+        """Sum the amount of DEPOSIT/WITHDRAWAL activities whose notes start
+        with ``notes_prefix``, signed (deposits positive, withdrawals negative).
+
+        Used by the Fidelity provider to compute the delta gains-offset:
+        ``current_gain - cumulative_existing_offset`` becomes the new
+        DEPOSIT to emit on each monthly run.
+        """
+        try:
+            resp = await self._request(
+                "POST", _ACTIVITIES_SEARCH,
+                json={"accountIds": [account_id], "page": 1, "pageSize": 500},
+            )
+        except Exception:
+            return Decimal(0)
+        if resp.status_code >= 400:
+            return Decimal(0)
+        payload = resp.json()
+        rows = payload.get("data", payload) if isinstance(payload, dict) else payload
+        if not isinstance(rows, list):
+            return Decimal(0)
+        total = Decimal(0)
+        for r in rows:
+            if not isinstance(r, dict):
+                continue
+            notes = r.get("comment") or r.get("notes") or ""
+            if not isinstance(notes, str) or not notes.startswith(notes_prefix):
+                continue
+            amt_raw = r.get("amount")
+            if amt_raw is None:
+                continue
+            try:
+                amt = Decimal(str(amt_raw))
+            except Exception:
+                continue
+            atype = (r.get("activityType") or r.get("activity_type") or "").upper()
+            if atype == "WITHDRAWAL":
+                total -= amt
+            else:
+                total += amt
+        return total
+
     # -- manual holdings snapshots --
 
     async def push_manual_snapshots(