refactor(backend): extract date range validation utilities

Issue #5: Date validation logic was duplicated across availability
and booking routes.

Changes:
- Add date_validation.py with shared utilities:
  - get_bookable_date_range: returns (min_date, max_date) tuple
  - validate_date_in_range: validates date with contextual errors
- Update routes/availability.py to use shared utilities
- Update routes/booking.py to use shared utilities
- Remove redundant _get_date_range_bounds and _get_bookable_date_range
- Error messages now include context (book, set availability, etc.)

backend/routes/availability.py

@@ -1,6 +1,6 @@
 """Availability routes for admin to manage booking availability."""
 
-from datetime import date, timedelta
+from datetime import date
 
 from fastapi import APIRouter, Depends, HTTPException, Query
 from sqlalchemy import and_, delete, select
@@ -8,6 +8,7 @@ from sqlalchemy.ext.asyncio import AsyncSession
 
 from auth import require_permission
 from database import get_db
+from date_validation import validate_date_in_range
 from models import Availability, Permission, User
 from schemas import (
     AvailabilityDay,
@@ -16,35 +17,10 @@ from schemas import (
     SetAvailabilityRequest,
     TimeSlot,
 )
-from shared_constants import MAX_ADVANCE_DAYS, MIN_ADVANCE_DAYS
 
 router = APIRouter(prefix="/api/admin/availability", tags=["availability"])
 
 
-def _get_date_range_bounds() -> tuple[date, date]:
-    """Get valid date range (MIN_ADVANCE_DAYS to MAX_ADVANCE_DAYS)."""
-    today = date.today()
-    min_date = today + timedelta(days=MIN_ADVANCE_DAYS)
-    max_date = today + timedelta(days=MAX_ADVANCE_DAYS)
-    return min_date, max_date
-
-
-def _validate_date_in_range(d: date, min_date: date, max_date: date) -> None:
-    """Validate a date is within the allowed range."""
-    if d < min_date:
-        raise HTTPException(
-            status_code=400,
-            detail=f"Cannot set availability for past dates. "
-            f"Earliest allowed: {min_date}",
-        )
-    if d > max_date:
-        raise HTTPException(
-            status_code=400,
-            detail=f"Cannot set more than {MAX_ADVANCE_DAYS} days ahead. "
-            f"Latest allowed: {max_date}",
-        )
-
-
 @router.get("", response_model=AvailabilityResponse)
 async def get_availability(
     from_date: date = Query(..., alias="from", description="Start date (inclusive)"),
@@ -94,8 +70,7 @@ async def set_availability(
     _current_user: User = Depends(require_permission(Permission.MANAGE_AVAILABILITY)),
 ) -> AvailabilityDay:
     """Set availability for a specific date. Replaces any existing availability."""
-    min_date, max_date = _get_date_range_bounds()
-    _validate_date_in_range(request.date, min_date, max_date)
+    validate_date_in_range(request.date, context="set availability")
 
     # Validate slots don't overlap
     sorted_slots = sorted(request.slots, key=lambda s: s.start_time)
@@ -142,14 +117,12 @@ async def copy_availability(
     _current_user: User = Depends(require_permission(Permission.MANAGE_AVAILABILITY)),
 ) -> AvailabilityResponse:
     """Copy availability from one day to multiple target days."""
-    min_date, max_date = _get_date_range_bounds()
-
     # Validate source date is in range
-    _validate_date_in_range(request.source_date, min_date, max_date)
+    validate_date_in_range(request.source_date, context="copy from")
 
     # Validate target dates
     for target_date in request.target_dates:
-        _validate_date_in_range(target_date, min_date, max_date)
+        validate_date_in_range(target_date, context="copy to")
 
     # Get source availability
     result = await db.execute(