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

src/lib/components/admin/Settings/Documents.svelte

@@ -1185,6 +1185,23 @@
 								</div>
 							{/if}
 
+							<div class="  mb-2.5 flex w-full justify-between">
+								<div class=" self-center text-xs font-medium">
+									{$i18n.t('Reranking Batch Size')}
+								</div>
+
+								<div class="">
+									<input
+										bind:value={RAGConfig.RAG_RERANKING_BATCH_SIZE}
+										type="number"
+										class=" bg-transparent text-center w-14 outline-none"
+										min="1"
+										max="16000"
+										step="1"
+									/>
+								</div>
+							</div>
+
 							<div class="  mb-2.5 flex w-full justify-between">
 								<div class=" self-center text-xs font-medium">{$i18n.t('Top K')}</div>
 								<div class="flex items-center relative">