fix(review): require graphics_limit>=1, type _index_document, cover rlimit branch

Address PR #852 round 2:

- config: DOCUMENT_PDF_GRAPHICS_LIMIT validator is now gte=1 (pymupdf4llm treats
  0 as "no cap", which would re-expose the OOM); documented the zero semantics
  and that the per-worker mem rlimit needs a pod restart to change.
- processor: annotate `_index_document -> bool | None` and document the contract
  so the `if indexed is False` check is explicit/type-checkable.
- tests: add the RLIM_INFINITY-hard branch assertion for _apply_mem_limit
  (soft==target, hard stays unbounded).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

nextcloud_mcp_server/vector/processor.py

@@ -311,10 +311,13 @@ async def process_document(
 
 async def _index_document(
     doc_task: DocumentTask, nc_client: NextcloudClient, qdrant_client
-):
+) -> bool | None:
     """
     Index a single document (called by process_document with retry).
 
+    Returns ``False`` when a permanent parse failure means nothing was indexed
+    (the caller must then skip the success metrics); ``None`` otherwise.
+
     Args:
         doc_task: Document task to index
         nc_client: Authenticated Nextcloud client