docs: update README and add PostgreSQL to stores documentation

- Remove PostgreSQL from README backends list to match main's simplified format
- Add docs/stores.md with PostgreSQL entry in distributed stores section
- Apply ruff formatting to PostgreSQL store

Co-authored-by: William Easton <strawgate@users.noreply.github.com>

Author: github-actions[bot]

README.md

@@ -16,8 +16,8 @@ This monorepo contains two libraries:
 
 ## Why use this library?
 
-- **Multiple backends**: DynamoDB, Elasticsearch, Memcached, MongoDB, PostgreSQL,
-  Redis, RocksDB, Valkey, and In-memory, Disk, etc
+- **Multiple backends**: DynamoDB, Elasticsearch, Memcached, MongoDB, Redis,
+  RocksDB, Valkey, and In-memory, Disk, etc
 - **TTL support**: Automatic expiration handling across all store types
 - **Type-safe**: Full type hints with Protocol-based interfaces
 - **Adapters**: Pydantic model support, raise-on-missing behavior, etc