feat(providers): add balldontlie http adapter with backend selection

README.md

@@ -155,8 +155,8 @@ Default auth routes:
 
 - Open `/admin/` -> `IngestionRun`
 - Use admin actions:
-  - `Queue full MVP sync`
-  - `Queue incremental MVP sync`
+  - `Queue full sync (default provider)`
+  - `Queue incremental sync (default provider)`
   - `Retry selected ingestion runs`
 
 ### Trigger from shell (manual)
@@ -167,7 +167,7 @@ docker compose exec web python manage.py shell
 
 ```python
 from apps.ingestion.tasks import trigger_full_sync
-trigger_full_sync.delay(provider_namespace="mvp_demo")
+trigger_full_sync.delay(provider_namespace="balldontlie")
 ```
 
 ### Logs and diagnostics
@@ -176,6 +176,16 @@ trigger_full_sync.delay(provider_namespace="mvp_demo")
 - Structured error records: `IngestionError`
 - Provider entity mappings + diagnostic payload snippets: `ExternalMapping`
 
+## Provider Backend Selection
+
+Provider backend is selected via environment variables:
+
+- `PROVIDER_BACKEND=demo` uses the local JSON fixture adapter (`mvp_demo`)
+- `PROVIDER_BACKEND=balldontlie` uses the HTTP adapter (`balldontlie`)
+- `PROVIDER_DEFAULT_NAMESPACE` can override backend mapping explicitly
+
+The balldontlie adapter is NBA-centric and intended as MVP ingestion only. The provider abstraction remains ready for future multi-league providers (for example Sportradar or FIBA GDAP).
+
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