Simplify unique constraint for documents (#1098)

This commit refined the project's document indexing strategy to
accommodate unique key constraints while handling soft deletions.
Previously, this was solved by applying partial index and unique
constraint, but partial index could not be used when sharding. By
adding the `removed_at` field into the uniqueness check, we now ensure
only one active document exists at a time.

server/backend/database/mongo/indexes.go

@@ -111,12 +111,9 @@ var collectionInfos = []collectionInfo{
 			Keys: bsonx.Doc{
 				{Key: "project_id", Value: bsonx.Int32(1)}, // shard key
 				{Key: "key", Value: bsonx.Int32(1)},
+				{Key: "removed_at", Value: bsonx.Int32(1)},
 			},
-			Options: options.Index().SetPartialFilterExpression(
-				bsonx.Doc{
-					{Key: "removed_at", Value: bsonx.Null()},
-				},
-			).SetUnique(true),
+			Options: options.Index().SetUnique(true),
 		}},
 	}, {
 		name: ColChanges,