Standardize timezone usage to timezone.utc

- Replaced all UTC imports with timezone imports
- Changed all datetime.now(UTC) to datetime.now(timezone.utc)
- Consistent with booking.py and more explicit about timezone usage
- Updated models.py, routes/auth.py, and routes/invites.py

backend/models.py

@@ -1,4 +1,4 @@
-from datetime import datetime, date, time, UTC
+from datetime import datetime, date, time, timezone
 from enum import Enum as PyEnum
 from typing import TypedDict
 from sqlalchemy import Integer, String, Float, DateTime, Date, Time, ForeignKey, Table, Column, Enum, UniqueConstraint, select
@@ -197,7 +197,7 @@ class SumRecord(Base):
     b: Mapped[float] = mapped_column(Float, nullable=False)
     result: Mapped[float] = mapped_column(Float, nullable=False)
     created_at: Mapped[datetime] = mapped_column(
-        DateTime(timezone=True), default=lambda: datetime.now(UTC)
+        DateTime(timezone=True), default=lambda: datetime.now(timezone.utc)
     )
 
 
@@ -209,7 +209,7 @@ class CounterRecord(Base):
     value_before: Mapped[int] = mapped_column(Integer, nullable=False)
     value_after: Mapped[int] = mapped_column(Integer, nullable=False)
     created_at: Mapped[datetime] = mapped_column(
-        DateTime(timezone=True), default=lambda: datetime.now(UTC)
+        DateTime(timezone=True), default=lambda: datetime.now(timezone.utc)
     )
 
 
@@ -244,7 +244,7 @@ class Invite(Base):
     
     # Timestamps
     created_at: Mapped[datetime] = mapped_column(
-        DateTime(timezone=True), default=lambda: datetime.now(UTC)
+        DateTime(timezone=True), default=lambda: datetime.now(timezone.utc)
     )
     spent_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True), nullable=True)
     revoked_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True), nullable=True)
@@ -262,12 +262,12 @@ class Availability(Base):
     start_time: Mapped[time] = mapped_column(Time, nullable=False)
     end_time: Mapped[time] = mapped_column(Time, nullable=False)
     created_at: Mapped[datetime] = mapped_column(
-        DateTime(timezone=True), default=lambda: datetime.now(UTC)
+        DateTime(timezone=True), default=lambda: datetime.now(timezone.utc)
     )
     updated_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True), 
-        default=lambda: datetime.now(UTC),
-        onupdate=lambda: datetime.now(UTC)
+        default=lambda: datetime.now(timezone.utc),
+        onupdate=lambda: datetime.now(timezone.utc)
     )
 
 
@@ -290,6 +290,6 @@ class Appointment(Base):
         Enum(AppointmentStatus), nullable=False, default=AppointmentStatus.BOOKED
     )
     created_at: Mapped[datetime] = mapped_column(
-        DateTime(timezone=True), default=lambda: datetime.now(UTC)
+        DateTime(timezone=True), default=lambda: datetime.now(timezone.utc)
     )
     cancelled_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True), nullable=True)