feat: add RAG_RERANKING_BATCH_SIZE configuration option

Add configurable reranker batch size (env var RAG_RERANKING_BATCH_SIZE,
default 32) following the same pattern as RAG_EMBEDDING_BATCH_SIZE.

- config.py: PersistentConfig for RAG_RERANKING_BATCH_SIZE
- main.py: import, state init, pass to get_reranking_function
- colbert.py: accept batch_size param in predict() (was hardcoded 32)
- utils.py: get_reranking_function passes batch_size at call time
- retrieval.py: expose in config GET/POST endpoints and ConfigForm
- Documents.svelte: add Reranking Batch Size input in admin settings

Closes #23730

backend/open_webui/config.py

@@ -2965,6 +2965,12 @@ RAG_RERANKING_MODEL_TRUST_REMOTE_CODE = (
     os.environ.get('RAG_RERANKING_MODEL_TRUST_REMOTE_CODE', 'True').lower() == 'true'
 )
 
+RAG_RERANKING_BATCH_SIZE = PersistentConfig(
+    'RAG_RERANKING_BATCH_SIZE',
+    'rag.reranking_batch_size',
+    int(os.environ.get('RAG_RERANKING_BATCH_SIZE', '32')),
+)
+
 RAG_EXTERNAL_RERANKER_URL = PersistentConfig(
     'RAG_EXTERNAL_RERANKER_URL',
     'rag.external_reranker_url',