feat(db): flow_link, metric_cash_flow_broker и price_coverage

Три таблицы одной миграцией, а не тремя: автогенерация из трёх параллельных веток
дала бы три ревизии с общим down_revision, то есть ручную разборку ветвления вместо
экономии.

price_coverage засевается прямо в миграции из price_daily: она отвечает на вопрос «с
какой даты мы УЖЕ спрашивали ISS», и без засева первый же прогон moex перекачал бы
историю всех 77 бумаг целиком.

flow_link_kind снимается на откате явно. DROP TABLE оставляет тип в базе, и следующий
upgrade упал бы на CREATE TYPE — то же, что уже сделано для остальных енумов домена.

backend/src/fintracker/models/metrics.py

@@ -48,6 +48,40 @@ class MetricCashFlowMonthly(Base):
     computed_at: Mapped[datetime] = mapped_column(server_default=func.now())
 
 
+class MetricCashFlowBroker(Base):
+    """Money put into and taken out of the brokerage accounts, per scope and month.
+
+    The sibling of `metric_cash_flow_monthly`, for the other half of the picture: that one
+    answers what the household earned and spent, this one what it moved across the portfolio
+    boundary. The scope key is the same string as everywhere else in the investment metrics
+    (`all`, `account:<id>`, `portfolio:<id>`).
+
+    Deposits and withdrawals are kept apart, both as positive magnitudes, because the screen
+    shows both bars: a month that took 200 000 ₽ in and 200 000 ₽ out is not the same month
+    as one that saw no money at all, and a single signed sum cannot tell them apart.
+    `net_rub = deposits_rub - withdrawals_rub` and equals the month's sum of
+    `metric_portfolio_value_daily.external_flow_rub` for the same scope.
+
+    A month with no flow gets no row at all — the series is sparse on purpose, so the client
+    can tell "nothing happened" from "zero on balance".
+    """
+
+    __tablename__ = "metric_cash_flow_broker"
+
+    scope: Mapped[str] = mapped_column(String(32), primary_key=True)
+    month: Mapped[date] = mapped_column(primary_key=True)
+    """First day of the month."""
+    deposits_rub: Mapped[Decimal]
+    """Everything that came in, positive: cash deposits and securities transferred in."""
+    withdrawals_rub: Mapped[Decimal]
+    """Everything that went out, also positive."""
+    net_rub: Mapped[Decimal]
+    """deposits - withdrawals; negative in a month that took more out than it put in."""
+    event_count: Mapped[int] = mapped_column(Integer, default=0)
+    """Flow events behind the row — the ones that converted; see `metric_data_quality`."""
+    computed_at: Mapped[datetime] = mapped_column(server_default=func.now())
+
+
 class MetricSpendingByCategory(Base):
     __tablename__ = "metric_spending_by_category"
     __table_args__ = (UniqueConstraint("month", "category_id", postgresql_nulls_not_distinct=True),)