feat: add DecisionRepository with unit tests

Repository provides upsert, get, delete, and disliked-ID-set queries
for listing decisions. All 6 unit tests pass.

repositories/decision_repository.py

@@ -0,0 +1,75 @@
+"""Repository for listing decisions (like/dislike)."""
+from datetime import datetime
+
+from models.decision import ListingDecision
+from sqlalchemy import Engine
+from sqlmodel import Session, select
+
+
+class DecisionRepository:
+    engine: Engine
+
+    def __init__(self, engine: Engine) -> None:
+        self.engine = engine
+
+    def upsert_decision(
+        self, user_id: int, listing_id: int, listing_type: str, decision: str
+    ) -> ListingDecision:
+        with Session(self.engine) as session:
+            statement = select(ListingDecision).where(
+                ListingDecision.user_id == user_id,
+                ListingDecision.listing_id == listing_id,
+                ListingDecision.listing_type == listing_type,
+            )
+            existing = session.exec(statement).first()
+            if existing:
+                existing.decision = decision
+                existing.updated_at = datetime.utcnow()
+                session.add(existing)
+                session.commit()
+                session.refresh(existing)
+                return existing
+            new_decision = ListingDecision(
+                user_id=user_id,
+                listing_id=listing_id,
+                listing_type=listing_type,
+                decision=decision,
+            )
+            session.add(new_decision)
+            session.commit()
+            session.refresh(new_decision)
+            return new_decision
+
+    def get_decisions_for_user(self, user_id: int) -> list[ListingDecision]:
+        with Session(self.engine) as session:
+            statement = select(ListingDecision).where(
+                ListingDecision.user_id == user_id
+            )
+            return list(session.exec(statement).all())
+
+    def delete_decision(
+        self, user_id: int, listing_id: int, listing_type: str
+    ) -> bool:
+        with Session(self.engine) as session:
+            statement = select(ListingDecision).where(
+                ListingDecision.user_id == user_id,
+                ListingDecision.listing_id == listing_id,
+                ListingDecision.listing_type == listing_type,
+            )
+            existing = session.exec(statement).first()
+            if existing is None:
+                return False
+            session.delete(existing)
+            session.commit()
+            return True
+
+    def get_disliked_listing_ids(
+        self, user_id: int, listing_type: str
+    ) -> set[int]:
+        with Session(self.engine) as session:
+            statement = select(ListingDecision.listing_id).where(
+                ListingDecision.user_id == user_id,
+                ListingDecision.listing_type == listing_type,
+                ListingDecision.decision == "disliked",
+            )
+            return {row for row in session.exec(statement).all()}