Add document ETag optimistic concurrency (#16)

* Add ETag optimistic concurrency for document replace

Co-authored-by: Pijus Kamandulis <[email protected]>

* Expose precondition error code header

Co-authored-by: Pijus Kamandulis <[email protected]>

* Stop Badger GC before closing datastore

Co-authored-by: Pijus Kamandulis <[email protected]>

---------

Co-authored-by: Cursor Agent <[email protected]>

internal/constants/responses.go

@@ -35,3 +35,7 @@ var UnknownErrorResponse = gin.H{"message": "Unknown error"}
 var NotFoundResponse = gin.H{"message": "NotFound"}
 var ConflictResponse = gin.H{"message": "Conflict"}
 var BadRequestResponse = gin.H{"message": "BadRequest"}
+var PreconditionFailedResponse = gin.H{
+	"code":    "PreconditionFailed",
+	"message": "Operation cannot be performed because one of the specified precondition is not met.",
+}