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/models/models.py

@@ -1,5 +1,7 @@
 import enum
 from datetime import date, datetime
+
+from app.timeutil import utcnow
 from typing import Optional
 
 from sqlalchemy import JSON, Column, UniqueConstraint
@@ -106,7 +108,7 @@ class AccountBase(SQLModel):
 
 class Account(AccountBase, table=True):
     id: Optional[int] = Field(default=None, primary_key=True)
-    updated_at: datetime = Field(default_factory=datetime.utcnow)
+    updated_at: datetime = Field(default_factory=utcnow)
 
 
 class AccountCreate(AccountBase):
@@ -147,7 +149,7 @@ class TransactionBase(SQLModel):
 
 class Transaction(TransactionBase, table=True):
     id: Optional[int] = Field(default=None, primary_key=True)
-    created_at: datetime = Field(default_factory=datetime.utcnow)
+    created_at: datetime = Field(default_factory=utcnow)
     category: Optional[Category] = Relationship(back_populates="transactions")
 
 
@@ -182,7 +184,7 @@ class ExchangeRate(SQLModel, table=True):
     date: datetime
     buy_rate: float
     sell_rate: float
-    fetched_at: datetime = Field(default_factory=datetime.utcnow)
+    fetched_at: datetime = Field(default_factory=utcnow)
 
 
 class ExchangeRateRead(SQLModel):
@@ -199,7 +201,7 @@ class APIToken(SQLModel, table=True):
     id: Optional[int] = Field(default=None, primary_key=True)
     name: str
     token_hash: str = Field(index=True)
-    created_at: datetime = Field(default_factory=datetime.utcnow)
+    created_at: datetime = Field(default_factory=utcnow)
     expires_at: Optional[datetime] = None
     is_active: bool = True
 
@@ -227,7 +229,7 @@ class UserSettings(SQLModel, table=True):
         default_factory=dict,
         sa_column=Column(JSON, nullable=False, server_default="{}"),
     )
-    updated_at: datetime = Field(default_factory=datetime.utcnow)
+    updated_at: datetime = Field(default_factory=utcnow)
 
 
 class UserSettingsRead(SQLModel):
@@ -262,7 +264,7 @@ class RecurringItemBase(SQLModel):
 
 class RecurringItem(RecurringItemBase, table=True):
     id: Optional[int] = Field(default=None, primary_key=True)
-    created_at: datetime = Field(default_factory=datetime.utcnow)
+    created_at: datetime = Field(default_factory=utcnow)
     category: Optional[Category] = Relationship()
 
 
@@ -298,7 +300,7 @@ class PushSubscription(SQLModel, table=True):
     endpoint: str = Field(unique=True)
     p256dh: str
     auth: str
-    created_at: datetime = Field(default_factory=datetime.utcnow)
+    created_at: datetime = Field(default_factory=utcnow)
 
 
 class PushSubscriptionCreate(SQLModel):
@@ -331,7 +333,7 @@ class PensionSnapshot(PensionSnapshotBase, table=True):
         UniqueConstraint("fund", "period_start", "period_end"),
     )
     id: Optional[int] = Field(default=None, primary_key=True)
-    created_at: datetime = Field(default_factory=datetime.utcnow)
+    created_at: datetime = Field(default_factory=utcnow)
 
 
 class PensionSnapshotRead(PensionSnapshotBase):
@@ -348,8 +350,8 @@ class BalanceOverride(SQLModel, table=True):
     year: int
     month: int
     override_balance: float
-    created_at: datetime = Field(default_factory=datetime.utcnow)
-    updated_at: datetime = Field(default_factory=datetime.utcnow)
+    created_at: datetime = Field(default_factory=utcnow)
+    updated_at: datetime = Field(default_factory=utcnow)
 
 
 class BalanceOverrideCreate(SQLModel):
@@ -379,7 +381,7 @@ class SavingsAccrualBase(SQLModel):
 class SavingsAccrual(SavingsAccrualBase, table=True):
     __table_args__ = (UniqueConstraint("year", "month"),)
     id: Optional[int] = Field(default=None, primary_key=True)
-    applied_at: datetime = Field(default_factory=datetime.utcnow)
+    applied_at: datetime = Field(default_factory=utcnow)
 
 
 class SavingsAccrualCreate(SavingsAccrualBase):
@@ -423,7 +425,7 @@ class MunicipalReceiptBase(SQLModel):
 class MunicipalReceipt(MunicipalReceiptBase, table=True):
     __table_args__ = (UniqueConstraint("account", "period"),)
     id: Optional[int] = Field(default=None, primary_key=True)
-    created_at: datetime = Field(default_factory=datetime.utcnow)
+    created_at: datetime = Field(default_factory=utcnow)
     water_readings: list["WaterMeterReading"] = Relationship(
         back_populates="receipt",
     )
@@ -458,7 +460,7 @@ class WaterMeterReadingBase(SQLModel):
 class WaterMeterReading(WaterMeterReadingBase, table=True):
     __table_args__ = (UniqueConstraint("meter_id", "period", "is_historical"),)
     id: Optional[int] = Field(default=None, primary_key=True)
-    created_at: datetime = Field(default_factory=datetime.utcnow)
+    created_at: datetime = Field(default_factory=utcnow)
     receipt: Optional[MunicipalReceipt] = Relationship(
         back_populates="water_readings",
     )