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/cli.py

@@ -438,10 +438,8 @@ def fidelity_ingest(
         sys.exit(2)
 
     async def _run() -> None:
-        from datetime import date as _date_t
-
         from broker_sync.providers.fidelity_planviewer import (
-            fidelity_holdings_to_snapshot,
+            gains_offset_delta_activity,
         )
 
         sink = WealthfolioSink(
@@ -461,37 +459,36 @@ def fidelity_ingest(
             result = await sync_provider_to_wealthfolio(
                 provider=provider, sink=sink, dedup=dedup, since=since,
             )
-            # PlanViewer has no historical per-fund unit-price feed, so
-            # the Activity stream above only carries cash flows. The
-            # current-pot fund positions captured in the same scrape get
-            # pushed via /api/v1/snapshots/import so per-fund quantity +
-            # cost basis land in WF (and propagate to the wealth
-            # dashboard's Positions table via pg-sync).
-            snapshot_imported = 0
+            # PlanViewer doesn't expose per-fund unit prices in any feed
+            # WF can consume, so the only way to keep WF's pension total in
+            # line with the live PlanViewer pot value is to emit a small
+            # DEPOSIT (or WITHDRAWAL on a market drop) each run sized to
+            # the growth since the last scrape. The dav_corrected PG view
+            # subtracts these offsets from net_contribution so the
+            # dashboard's Growth/ROI panels stay accurate.
+            gains_delta_emitted = 0
             if provider.last_holdings:
-                snapshot = fidelity_holdings_to_snapshot(
+                wf_account_id = await sink.ensure_account(provider.accounts()[0])
+                prior_offset = await sink.cumulative_amount_with_notes_prefix(
+                    account_id=wf_account_id,
+                    notes_prefix="fidelity-planviewer:unrealised-gains-offset",
+                )
+                delta = gains_offset_delta_activity(
                     holdings=provider.last_holdings,
                     total_real_contribution=provider.last_total_contribution,
-                    as_of=_date_t.today(),
+                    prior_offset_cumulative=prior_offset,
+                    as_of=datetime.now(UTC),
                 )
-                if snapshot is not None:
-                    # /api/v1/snapshots/import wants WF's own account UUID,
-                    # not our logical provider id — look it up via the same
-                    # match the pipeline used (provider+providerAccountId).
-                    wf_account_id = await sink.ensure_account(
-                        provider.accounts()[0],
-                    )
-                    push_result = await sink.push_manual_snapshots(
-                        account_id=wf_account_id, snapshots=[snapshot],
-                    )
-                    snapshot_imported = int(push_result.get("snapshotsImported", 0))
+                if delta is not None:
+                    await sink.import_activities([delta])
+                    gains_delta_emitted = 1
         finally:
             await sink.close()
         typer.echo(f"fidelity-ingest: fetched={result.fetched} "
                    f"new={result.new_after_dedup} "
                    f"imported={result.imported} "
                    f"failed={result.failed} "
-                   f"snapshots={snapshot_imported}")
+                   f"gains_delta={gains_delta_emitted}")
         if result.failed > 0:
             sys.exit(1)