chore: import upstream snapshot with attribution

store/migration/sqlite/0.28/00__user_identity.sql

@@ -0,0 +1,15 @@
+-- user_identity stores the linkage between an external identity subject and a local user.
+-- (provider, extern_uid) is unique across the table; provider stores the idp.uid.
+-- Each local user can link at most one external account per provider.
+CREATE TABLE user_identity (
+  id         INTEGER PRIMARY KEY AUTOINCREMENT,
+  user_id    INTEGER NOT NULL,
+  provider   TEXT    NOT NULL,
+  extern_uid TEXT    NOT NULL,
+  created_ts BIGINT  NOT NULL DEFAULT (strftime('%s', 'now')),
+  updated_ts BIGINT  NOT NULL DEFAULT (strftime('%s', 'now')),
+  UNIQUE (provider, extern_uid),
+  UNIQUE (user_id, provider)
+);
+
+CREATE INDEX idx_user_identity_user_id ON user_identity(user_id);