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/schemas.py

@@ -1,8 +1,8 @@
 """Pydantic schemas for API request/response models."""
-from datetime import datetime
+from datetime import datetime, date, time
 from typing import Generic, TypeVar
 
-from pydantic import BaseModel, EmailStr
+from pydantic import BaseModel, EmailStr, field_validator
 
 
 class UserCredentials(BaseModel):
@@ -140,6 +140,49 @@ class AdminUserResponse(BaseModel):
     email: str
 
 
+# =============================================================================
+# Availability Schemas
+# =============================================================================
+
+class TimeSlot(BaseModel):
+    """A single time slot (start and end time)."""
+    start_time: time
+    end_time: time
+    
+    @field_validator("start_time", "end_time")
+    @classmethod
+    def validate_15min_boundary(cls, v: time) -> time:
+        """Ensure times are on 15-minute boundaries."""
+        if v.minute not in (0, 15, 30, 45):
+            raise ValueError("Time must be on 15-minute boundary (:00, :15, :30, :45)")
+        if v.second != 0 or v.microsecond != 0:
+            raise ValueError("Time must not have seconds or microseconds")
+        return v
+
+
+class AvailabilityDay(BaseModel):
+    """Availability for a single day."""
+    date: date
+    slots: list[TimeSlot]
+
+
+class AvailabilityResponse(BaseModel):
+    """Response model for availability query."""
+    days: list[AvailabilityDay]
+
+
+class SetAvailabilityRequest(BaseModel):
+    """Request to set availability for a specific date."""
+    date: date
+    slots: list[TimeSlot]
+
+
+class CopyAvailabilityRequest(BaseModel):
+    """Request to copy availability from one day to others."""
+    source_date: date
+    target_dates: list[date]
+
+
 # =============================================================================
 # Meta/Constants Schemas
 # =============================================================================