Fix ReverseLowerBound behavior in MemDB with unique index (#1074)

This change resolves a test failure in the js-sdk caused by an unexpected
MemDB search behavior. The issue stems from the `ReverseLowerBound` method
behaving differently for unique and non-unique indexes.

Key changes:
- Set `doc_id_actor_id_server_seq` index as unique
- Ensures correct record retrieval in `FindMinSyncedSeqInfo`
- Resolves `ConnectError: [internal] change not found` during DetachDocument

Addressed a known MemDB limitation where index uniqueness affects search
behavior, referencing hashicorp/go-memdb#96.

server/backend/database/memory/indexes.go

@@ -168,7 +168,8 @@ var schema = &memdb.DBSchema{
 					},
 				},
 				"doc_id_actor_id_server_seq": {
-					Name: "doc_id_actor_id_server_seq",
+					Name:   "doc_id_actor_id_server_seq",
+					Unique: true,
 					Indexer: &memdb.CompoundIndex{
 						Indexes: []memdb.Indexer{
 							&memdb.StringFieldIndex{Field: "DocID"},