Task 2: guard SeriesApp NFOService init on NFOServiceFactory fallback and document config-only TMDB API key support

docs/CONFIGURATION.md

@@ -217,6 +217,7 @@ Source: [src/server/models/config.py](../src/server/models/config.py#L15-L24)
 - `auto_create` creates NFO files during the download process
 - `update_on_scan` refreshes metadata when scanning existing anime
 - Image downloads require valid `tmdb_api_key`
+- `TMDB_API_KEY` environment variable is optional when `nfo.tmdb_api_key` is configured in `data/config.json`
 - Larger image sizes (`w780`, `original`) consume more storage space
 
 Source: [src/server/models/config.py](../src/server/models/config.py#L109-L132)

docs/tasks.md

@@ -54,6 +54,8 @@ Up to 5 background workers share one `NFOService`/`TMDBClient` instance. The `as
 
 ## Task 2 — Guard NFOService init in SeriesApp on factory fallback, not just env var
 
+- [x] Completed
+
 ### Where
 `src/core/SeriesApp.py` — `__init__` method (~line 175)