feat(db): add memory storage driver

removes the sqlite dependency for tests, also brings back the option for
users to run zero persistence instances of tinyauth.

adds new mapErr fn for sqlc wrapper gen to prevent sql errors from
leaking out of the store implementation.

internal/repository/store.go

@@ -1,6 +1,12 @@
 package repository
 
-import "context"
+import (
+	"context"
+	"errors"
+)
+
+// ErrNotFound is returned by Store methods when the requested record does not exist.
+var ErrNotFound = errors.New("not found")
 
 // Store is the interface that all storage drivers must implement.
 // The sqlc-generated *Queries struct satisfies this interface for SQLite.