feat: add decision_service with unit tests

Service layer provides validation, delegation to repository, and
get_disliked_listing_ids for filtering. All 7 unit tests pass.

services/decision_service.py

@@ -0,0 +1,54 @@
+"""Unified decision service -- shared between CLI and HTTP API."""
+from models.decision import ListingDecision
+from repositories.decision_repository import DecisionRepository
+
+VALID_DECISIONS = {"liked", "disliked"}
+
+
+def set_decision(
+    repository: DecisionRepository,
+    user_id: int,
+    listing_id: int,
+    listing_type: str,
+    decision: str,
+) -> ListingDecision:
+    if decision not in VALID_DECISIONS:
+        raise ValueError(
+            f"Invalid decision '{decision}'. Must be one of: {VALID_DECISIONS}"
+        )
+    return repository.upsert_decision(
+        user_id=user_id,
+        listing_id=listing_id,
+        listing_type=listing_type,
+        decision=decision,
+    )
+
+
+def get_decisions(
+    repository: DecisionRepository,
+    user_id: int,
+) -> list[ListingDecision]:
+    return repository.get_decisions_for_user(user_id)
+
+
+def clear_decision(
+    repository: DecisionRepository,
+    user_id: int,
+    listing_id: int,
+    listing_type: str,
+) -> bool:
+    return repository.delete_decision(
+        user_id=user_id,
+        listing_id=listing_id,
+        listing_type=listing_type,
+    )
+
+
+def get_disliked_listing_ids(
+    repository: DecisionRepository,
+    user_id: int,
+    listing_type: str,
+) -> set[int]:
+    return repository.get_disliked_listing_ids(
+        user_id=user_id, listing_type=listing_type
+    )