Replace deprecated datetime.utcnow with shared naive-UTC helper

All 29 call sites now use app.timeutil.utcnow(), which returns naive
UTC via the non-deprecated API. Semantics are unchanged on purpose:
every datetime column is TIMESTAMP WITHOUT TIME ZONE, so going aware
here would poison naive/aware comparisons. The TIMESTAMPTZ migration
(Phase 2) now has a single place to change. (BE-01)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/app/services/exchange_rate.py

@@ -8,6 +8,7 @@ from sqlalchemy import case
 from sqlmodel import Session, col, select
 
 from app.config import settings
+from app.timeutil import utcnow
 from app.db import engine
 from app.models.models import ExchangeRate
 
@@ -136,7 +137,7 @@ def _fetch_rate_from_apis() -> tuple[float, float] | None:
 def _remember(rate: ExchangeRate) -> ExchangeRate:
     """Store rate in both TTL cache and permanent last-known holder."""
     global _last_known
-    _cache["current"] = (rate, datetime.utcnow())
+    _cache["current"] = (rate, utcnow())
     _last_known = rate
     return rate
 
@@ -147,11 +148,11 @@ def get_current_rate(session: Session) -> ExchangeRate | None:
 
     # 1. Fresh memory cache (< 1 hour)
     cached = _cache.get("current")
-    if cached and datetime.utcnow() - cached[1] < CACHE_TTL:
+    if cached and utcnow() - cached[1] < CACHE_TTL:
         return cached[0]
 
     # 2. Fresh DB rate (< 1 hour)
-    one_hour_ago = datetime.utcnow() - CACHE_TTL
+    one_hour_ago = utcnow() - CACHE_TTL
     db_rate = session.exec(
         select(ExchangeRate)
         .where(ExchangeRate.fetched_at > one_hour_ago)
@@ -164,7 +165,7 @@ def get_current_rate(session: Session) -> ExchangeRate | None:
     result = _fetch_rate_from_apis()
     if result is not None:
         buy, sell = result
-        rate = ExchangeRate(date=datetime.utcnow(), buy_rate=buy, sell_rate=sell)
+        rate = ExchangeRate(date=utcnow(), buy_rate=buy, sell_rate=sell)
         session.add(rate)
         session.commit()
         session.refresh(rate)
@@ -230,7 +231,7 @@ def get_crc_rate(code: str) -> float | None:
         return 1.0
 
     cached = _xcrc_cache.get(code)
-    if cached and datetime.utcnow() - cached[1] < CACHE_TTL:
+    if cached and utcnow() - cached[1] < CACHE_TTL:
         return cached[0]
 
     if code in _COINGECKO_IDS:
@@ -239,7 +240,7 @@ def get_crc_rate(code: str) -> float | None:
         rate = _fetch_fiat_crc_mid(code)
 
     if rate is not None:
-        _xcrc_cache[code] = (rate, datetime.utcnow())
+        _xcrc_cache[code] = (rate, utcnow())
         _last_known_xcrc[code] = rate
         return rate
 
@@ -293,7 +294,7 @@ def _refresh_usd_rate() -> bool:
         return False
     buy, sell = fetched
     with Session(engine) as session:
-        rate = ExchangeRate(date=datetime.utcnow(), buy_rate=buy, sell_rate=sell)
+        rate = ExchangeRate(date=utcnow(), buy_rate=buy, sell_rate=sell)
         session.add(rate)
         session.commit()
         session.refresh(rate)
@@ -309,7 +310,7 @@ def _refresh_other_rate(code: str) -> bool:
         rate = _fetch_fiat_crc_mid(code)
     if rate is None:
         return False
-    _xcrc_cache[code] = (rate, datetime.utcnow())
+    _xcrc_cache[code] = (rate, utcnow())
     _last_known_xcrc[code] = rate
     return True
 
@@ -368,7 +369,7 @@ async def refresh_rates_periodically(
 
 def get_rate_history(session: Session, days: int = 30) -> list[ExchangeRate]:
     """Get historical exchange rates."""
-    cutoff = datetime.utcnow() - timedelta(days=days)
+    cutoff = utcnow() - timedelta(days=days)
     return list(
         session.exec(
             select(ExchangeRate)