store: add users table and user_id scoping to migrations

Schema-only step toward per-user profile isolation: profile/workout_kinds/
sync_state are rebuilt to drop their singleton constraints, activities/
sync_runs gain a nullable user_id column. Store methods are scoped in
later tasks.

backend/internal/store/migrations/0021_users_table.sql

@@ -0,0 +1,6 @@
+CREATE TABLE users (
+    id           INTEGER PRIMARY KEY AUTOINCREMENT,
+    oidc_sub     TEXT NOT NULL UNIQUE,
+    display_name TEXT NOT NULL,
+    created_at   TEXT NOT NULL DEFAULT (datetime('now'))
+);