feat(authz): add database-backed PermissionStore

Adds dbPermissionStore/ensureAuthzTables/authzMigrationSql, backed by
_wrn_authz_assignment and _wrn_authz_grant tables, plus a ./db subpath
export. Passes the identical 19-test store-conformance suite the memory
adapter passes, including tenant-scope isolation.

packages/authz/package.json

@@ -5,6 +5,7 @@
   "type": "module",
   "main": "src/index.ts",
   "exports": {
-    ".": "./src/index.ts"
+    ".": "./src/index.ts",
+    "./db": "./src/db.ts"
   }
 }