Move slot expansion logic to ExchangeService

- Add get_available_slots() and _expand_availability_to_slots() to ExchangeService
- Update routes/exchange.py to use ExchangeService.get_available_slots()
- Remove all business logic from get_available_slots endpoint
- Add AvailabilityRepository to ExchangeService dependencies
- Add Availability and BookableSlot imports to ExchangeService
- Fix import path for validate_date_in_range (use date_validation module)
- Remove unused user_repo variable and import from routes/invites.py
- Fix mypy error in ValidationError by adding proper type annotation

backend/repositories/invite.py

@@ -2,6 +2,7 @@
 
 from sqlalchemy import desc, func, select
 from sqlalchemy.ext.asyncio import AsyncSession
+from sqlalchemy.orm import joinedload
 
 from models import Invite, InviteStatus
 
@@ -13,22 +14,32 @@ class InviteRepository:
         self.db = db
 
     async def get_by_identifier(self, identifier: str) -> Invite | None:
-        """Get an invite by identifier."""
+        """Get an invite by identifier, eagerly loading relationships."""
         result = await self.db.execute(
-            select(Invite).where(Invite.identifier == identifier)
+            select(Invite)
+            .options(joinedload(Invite.godfather), joinedload(Invite.used_by))
+            .where(Invite.identifier == identifier)
         )
         return result.scalar_one_or_none()
 
     async def get_by_id(self, invite_id: int) -> Invite | None:
-        """Get an invite by ID."""
-        result = await self.db.execute(select(Invite).where(Invite.id == invite_id))
+        """Get an invite by ID, eagerly loading relationships."""
+        result = await self.db.execute(
+            select(Invite)
+            .options(joinedload(Invite.godfather), joinedload(Invite.used_by))
+            .where(Invite.id == invite_id)
+        )
         return result.scalar_one_or_none()
 
     async def get_by_godfather_id(
         self, godfather_id: int, order_by_desc: bool = True
     ) -> list[Invite]:
-        """Get all invites for a godfather user."""
-        query = select(Invite).where(Invite.godfather_id == godfather_id)
+        """Get all invites for a godfather user, eagerly loading relationships."""
+        query = (
+            select(Invite)
+            .options(joinedload(Invite.used_by))
+            .where(Invite.godfather_id == godfather_id)
+        )
         if order_by_desc:
             query = query.order_by(desc(Invite.created_at))
         else:
@@ -57,9 +68,11 @@ class InviteRepository:
         status: InviteStatus | None = None,
         godfather_id: int | None = None,
     ) -> list[Invite]:
-        """Get paginated list of invites."""
+        """Get paginated list of invites, eagerly loading relationships."""
         offset = (page - 1) * per_page
-        query = select(Invite)
+        query = select(Invite).options(
+            joinedload(Invite.godfather), joinedload(Invite.used_by)
+        )
         if status:
             query = query.where(Invite.status == status)
         if godfather_id: