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/ledger.py

@@ -180,3 +180,43 @@ class LotDisposal(Base):
     holding_days: Mapped[int] = mapped_column(Integer)
     ldv_eligible: Mapped[bool]
     """Held 3+ years on an exchange-traded instrument (art. 219.1 NK)."""
+
+
+class FlowLinkKind(enum.StrEnum):
+    auto = "auto"
+    """Produced by `ledger/matching.py`; rebuilt from scratch on every refresh."""
+    manual = "manual"
+    """Confirmed by the user through the API; never touched by the matcher."""
+
+
+class FlowLink(TimestampMixin, Base):
+    """One ZenMoney transfer tied to the broker deposit/withdrawal it actually was (plan §1.6 C).
+
+    Without this pairing the same money is counted twice — once as the balance of the ZenMoney
+    account that mirrors the broker, once as the broker's own cash — and a top-up looks like an
+    expense in the cash flow. The unique constraints on both sides are what make the link a
+    1:1 statement: one transfer, one broker event, never a fan-out.
+
+    The scoring fields are kept because a link is a *guess*: `amount_delta` and `day_gap` are
+    what the reviewer needs in `GET /links/unmatched` to tell a good pairing from a lucky one.
+    """
+
+    __tablename__ = "flow_link"
+
+    id: Mapped[int] = mapped_column(primary_key=True)
+    cash_txn_id: Mapped[int] = mapped_column(
+        ForeignKey("cash_txn.id", ondelete="CASCADE"), unique=True
+    )
+    event_id: Mapped[int] = mapped_column(ForeignKey("event.id", ondelete="CASCADE"), unique=True)
+    kind: Mapped[FlowLinkKind] = mapped_column(
+        db_enum(FlowLinkKind, "flow_link_kind"), default=FlowLinkKind.auto
+    )
+    confidence: Mapped[Decimal]
+    """0..1; 1 means same day and the same kopeck."""
+    amount_delta: Mapped[Decimal]
+    """|ZenMoney amount - broker amount|, in `currency` — both sides share it by construction."""
+    currency: Mapped[str] = mapped_column(String(3))
+    day_gap: Mapped[int] = mapped_column(Integer)
+    """Business days between the two dates: money does not reach a broker over a weekend."""
+    note: Mapped[str | None] = mapped_column(Text)
+    """Why the pair was accepted (which route identified the broker account), for debugging."""