feat(saves): add slot-based save sync with content hash deduplication

- Add device registration and save synchronization
- Implement slot-based save organization with datetime tagging
- Add conflict detection for multi-device sync scenarios
- Add content hash computation for save deduplication
- Support ZIP inner-file hashing for consistent deduplication
- Add confirm_download endpoint for sync state management
- Add overwrite parameter to bypass conflict checks

backend/handler/database/saves_handler.py

@@ -1,6 +1,6 @@
 from collections.abc import Sequence
 
-from sqlalchemy import and_, delete, select, update
+from sqlalchemy import and_, delete, desc, select, update
 from sqlalchemy.orm import QueryableAttribute, Session, load_only
 
 from decorators.database import begin_session
@@ -42,12 +42,28 @@ class DBSavesHandler(DBBaseHandler):
             .limit(1)
         ).first()
 
+    @begin_session
+    def get_save_by_content_hash(
+        self,
+        user_id: int,
+        rom_id: int,
+        content_hash: str,
+        session: Session = None,  # type: ignore
+    ) -> Save | None:
+        return session.scalar(
+            select(Save)
+            .filter_by(rom_id=rom_id, user_id=user_id, content_hash=content_hash)
+            .limit(1)
+        )
+
     @begin_session
     def get_saves(
         self,
         user_id: int,
         rom_id: int | None = None,
         platform_id: int | None = None,
+        slot: str | None = None,
+        order_by_updated_at_desc: bool = False,
         only_fields: Sequence[QueryableAttribute] | None = None,
         session: Session = None,  # type: ignore
     ) -> Sequence[Save]:
@@ -61,6 +77,12 @@ class DBSavesHandler(DBBaseHandler):
                 Rom.platform_id == platform_id
             )
 
+        if slot is not None:
+            query = query.filter(Save.slot == slot)
+
+        if order_by_updated_at_desc:
+            query = query.order_by(desc(Save.updated_at))
+
         if only_fields:
             query = query.options(load_only(*only_fields))
 
@@ -125,3 +147,28 @@ class DBSavesHandler(DBBaseHandler):
         )
 
         return missing_saves
+
+    @begin_session
+    def get_saves_summary(
+        self,
+        user_id: int,
+        rom_id: int,
+        session: Session = None,  # type: ignore
+    ) -> dict:
+        saves = session.scalars(
+            select(Save)
+            .filter_by(user_id=user_id, rom_id=rom_id)
+            .order_by(desc(Save.updated_at))
+        ).all()
+
+        slots_data: dict[str | None, dict] = {}
+        for save in saves:
+            slot_key = save.slot
+            if slot_key not in slots_data:
+                slots_data[slot_key] = {"slot": slot_key, "count": 0, "latest": save}
+            slots_data[slot_key]["count"] += 1
+
+        return {
+            "total_count": len(saves),
+            "slots": list(slots_data.values()),
+        }