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/booking.py

@@ -11,6 +11,7 @@ from sqlalchemy.orm import joinedload
 
 from auth import require_permission
 from database import get_db
+from date_validation import validate_date_in_range
 from models import Appointment, AppointmentStatus, Availability, Permission, User
 from pagination import calculate_offset, create_paginated_response
 from schemas import (
@@ -20,7 +21,7 @@ from schemas import (
     BookingRequest,
     PaginatedAppointments,
 )
-from shared_constants import MAX_ADVANCE_DAYS, MIN_ADVANCE_DAYS, SLOT_DURATION_MINUTES
+from shared_constants import SLOT_DURATION_MINUTES
 
 router = APIRouter(prefix="/api/booking", tags=["booking"])
 
@@ -62,29 +63,9 @@ def _get_valid_minute_boundaries() -> tuple[int, ...]:
     return tuple(boundaries)
 
 
-def _get_bookable_date_range() -> tuple[date, date]:
-    """Get the valid date range for booking (tomorrow to +30 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_booking_date(d: date) -> None:
     """Validate a date is within the bookable range."""
-    min_date, max_date = _get_bookable_date_range()
-    if d < min_date:
-        raise HTTPException(
-            status_code=400,
-            detail=f"Cannot book for today or past dates. "
-            f"Earliest bookable: {min_date}",
-        )
-    if d > max_date:
-        raise HTTPException(
-            status_code=400,
-            detail=f"Cannot book more than {MAX_ADVANCE_DAYS} days ahead. "
-            f"Latest bookable: {max_date}",
-        )
+    validate_date_in_range(d, context="book")
 
 
 def _expand_availability_to_slots(