feat(cbr): USD и CNY тянутся всегда, новая валюта — с начала леджера

Валюта, впервые появившаяся в ZenMoney между двумя запусками, раньше показывала «нет курса» до следующего синка. Для валюты без сохранённых курсов история берётся с начала леджера, а не от общего курсора.

backend/src/fintracker/sources/cbr/sync.py

@@ -1,8 +1,14 @@
 """The `cbr` source: official RUB rates for every currency the ledger actually uses.
 
-What to fetch is derived from the data, not configured: the distinct currencies of
-`account.currency`, `cash_txn.income_currency` and `cash_txn.outcome_currency` (RUB itself
-is quoted as 1.0 by `pricing/fx.py`, so it is never requested).
+What to fetch: `ALWAYS_QUOTED` (the dollar and the yuan) whatever the data says, plus every
+currency the data uses — the distinct currencies of `account.currency`, `cash_txn.income_currency`
+and `cash_txn.outcome_currency`. RUB itself is quoted as 1.0 by `pricing/fx.py`, so it is never
+requested. The always-quoted pair exists because a currency that first appears in ZenMoney
+between two runs would otherwise show «нет курса» until the next run picked it up.
+
+A currency with no stored rates yet is fetched from the start of the ledger, not from the shared
+cursor: the cursor only knows what the OTHER currencies already have, and a new one needs the rates
+for every past transaction it appears in.
 
 Date range: from the earliest transaction minus 7 days (or today − 30 days when there are
 no transactions yet) up to today — `analytics.today_local()`, the deployment timezone, the
@@ -41,6 +47,8 @@ CURSOR_OVERLAP_DAYS = 3
 BACKFILL_TOLERANCE_DAYS = 14
 """How far the first stored quote may legitimately sit after the wanted start (holidays)."""
 NO_HISTORY_DAYS = 30
+ALWAYS_QUOTED = ("USD", "CNY")
+"""Fetched every run, in use or not: the two foreign currencies the ledger meets first."""
 CHUNK = 500
 
 
@@ -50,16 +58,9 @@ class CbrSource:
     async def sync(self, ctx: SyncContext) -> SyncResult:
         session = ctx.session
         today = today_local()
-        currencies = await currencies_in_use(session)
-        if not currencies:
-            log.info("cbr: only %s in use, nothing to fetch", BASE)
-            return SyncResult(
-                cursor_after=today.isoformat(),
-                counts={"currencies": 0, "rates": 0},
-                changed=False,
-            )
-
-        start = await _start_date(session, ctx.cursor_before, today)
+        currencies = sorted(set(await currencies_in_use(session)) | set(ALWAYS_QUOTED))
+        full_start = await _full_start(session, today)
+        start = await _start_date(session, ctx.cursor_before, today, full_start)
         warnings: list[str] = []
         stored = 0
         async with CbrClient() as client:
@@ -70,7 +71,11 @@ class CbrSource:
                 if not cbr_id:
                     warnings.append(f"{ccy} is not quoted by CBR — skipped (crypto or metal?)")
                     continue
-                quotes = await client.quotes(cbr_id, start, today)
+                # a currency the table has never seen needs the whole history, cursor or not
+                have = await session.scalar(
+                    select(RawCbrRate.ccy).where(RawCbrRate.ccy == ccy).limit(1)
+                )
+                quotes = await client.quotes(cbr_id, start if have else full_start, today)
                 stored += await _store(session, ccy, quotes)
                 fetched_currencies += 1
 
@@ -104,13 +109,19 @@ async def currencies_in_use(session: AsyncSession) -> list[str]:
     return sorted(code for code in found if code != BASE)
 
 
-async def _start_date(session: AsyncSession, cursor: str | None, today: date) -> date:
+async def _full_start(session: AsyncSession, today: date) -> date:
+    """Where the full history begins: a week before the first transaction, else 30 days back."""
     first_txn, _ = await ledger_date_range(session)
-    full_start = (
+    return (
         first_txn - timedelta(days=LOOKBACK_DAYS)
         if first_txn is not None
         else today - timedelta(days=NO_HISTORY_DAYS)
     )
+
+
+async def _start_date(
+    session: AsyncSession, cursor: str | None, today: date, full_start: date
+) -> date:
     cursor_date = _parse_date(cursor)
     if cursor_date is None:
         return full_start