Apply the Cin-1 / Cin-2 pattern from the download refactor PR to the
media server engine PR before review.
Cin-1 — explicit inheritance:
- PlexClient, JellyfinClient, NavidromeClient, SoulSyncClient now
explicitly inherit MediaServerClient instead of relying on
structural typing alone. Pre-change a reader of plex_client.py
had no way to know the class was supposed to satisfy the contract.
- Removed the engine + registry re-exports from
core/media_server/__init__.py to break the circular import that
the inheritance change introduced (importing the package now
triggered a chain that loaded clients before their base class
resolved). Submodules import directly: from
core.media_server.engine import MediaServerEngine, etc.
- Conformance test now also asserts isinstance() / issubclass()
against MediaServerClient — drift in any class fails at the test
boundary instead of at runtime.
Cin-2 — generic accessors + singleton:
- engine.configured_clients() — replaces the legacy per-server
`if X and X.is_connected(): clients[name] = X` chains in
web_server.py.
- engine.reload_config(name=None) — generic dispatch, so callers
pass the server name instead of reaching for plex_client.reload_config()
directly.
- get_media_server_engine() / set_media_server_engine() singleton
factory matching the get_metadata_engine() / get_download_orchestrator()
shape. web_server.py boots via set_media_server_engine(...) so
factory + global handle share state.
- 7 new tests pin the accessors + singleton behaviour.
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