Phase 1: Add Availability model and API

- Create Availability model with date, start_time, end_time
- Add availability schemas with 15-minute boundary validation
- Add admin endpoints:
  - GET /api/admin/availability - query by date range
  - PUT /api/admin/availability - set slots for a date
  - POST /api/admin/availability/copy - copy to multiple days
- Add 26 tests covering permissions, CRUD, and validation

backend/models.py

@@ -1,7 +1,7 @@
-from datetime import datetime, UTC
+from datetime import datetime, date, time, UTC
 from enum import Enum as PyEnum
 from typing import TypedDict
-from sqlalchemy import Integer, String, Float, DateTime, ForeignKey, Table, Column, Enum, select
+from sqlalchemy import Integer, String, Float, DateTime, Date, Time, ForeignKey, Table, Column, Enum, UniqueConstraint, select
 from sqlalchemy.orm import Mapped, mapped_column, relationship
 from sqlalchemy.ext.asyncio import AsyncSession
 from database import Base
@@ -248,3 +248,24 @@ class Invite(Base):
     )
     spent_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True), nullable=True)
     revoked_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True), nullable=True)
+
+
+class Availability(Base):
+    """Admin availability slots for booking."""
+    __tablename__ = "availability"
+    __table_args__ = (
+        UniqueConstraint("date", "start_time", name="uq_availability_date_start"),
+    )
+
+    id: Mapped[int] = mapped_column(Integer, primary_key=True, autoincrement=True)
+    date: Mapped[date] = mapped_column(Date, nullable=False, index=True)
+    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)
+    )
+    updated_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), 
+        default=lambda: datetime.now(UTC),
+        onupdate=lambda: datetime.now(UTC)
+    )