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/date_validation.py

@@ -0,0 +1,52 @@
+"""Date range validation utilities for booking and availability."""
+
+from datetime import date, timedelta
+
+from fastapi import HTTPException
+
+from shared_constants import MAX_ADVANCE_DAYS, MIN_ADVANCE_DAYS
+
+
+def get_bookable_date_range() -> tuple[date, date]:
+    """
+    Get the valid date range for booking/availability.
+
+    Returns:
+        Tuple of (min_date, max_date) where:
+        - min_date is MIN_ADVANCE_DAYS from today
+        - max_date is MAX_ADVANCE_DAYS from today
+    """
+    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,
+    *,
+    context: str = "action",
+) -> None:
+    """
+    Validate a date is within the bookable range.
+
+    Args:
+        d: The date to validate
+        context: Description for error messages (e.g., "book", "set availability")
+
+    Raises:
+        HTTPException: If the date is outside the valid range
+    """
+    min_date, max_date = get_bookable_date_range()
+
+    if d < min_date:
+        raise HTTPException(
+            status_code=400,
+            detail=f"Cannot {context} for past dates. Earliest allowed: {min_date}",
+        )
+    if d > max_date:
+        raise HTTPException(
+            status_code=400,
+            detail=f"Cannot {context} more than {MAX_ADVANCE_DAYS} days ahead. "
+            f"Latest allowed: {max_date}",
+        )