docs: add SetupService to architecture, update changelog and testing docs

- ARCHITECTURE.md: add setup_service.py to services list
- CHANGELOG.md: add Unreleased section with folder scan key resolution fix
- TESTING.md: add SetupService testing section with example tests

docs/ARCHITECTURE.md

@@ -81,6 +81,7 @@ src/server/
 |   +-- websocket_service.py# WebSocket broadcasting
 |   +-- queue_repository.py # Database persistence
 |   +-- nfo_service.py      # NFO metadata management
+|   +-- setup_service.py    # Series key resolution from folder names
 |   +-- folder_scan_service.py # Daily folder maintenance scan
 +-- models/                 # Pydantic models
 |   +-- auth.py             # Auth request/response models