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>

tests/unit/test_pdf_parse_isolation.py

@@ -129,6 +129,24 @@ def test_apply_mem_limit_caps_soft_below_finite_hard(monkeypatch):
     assert hard == 4 * 1024**3
 
 
+def test_apply_mem_limit_uses_target_when_hard_unlimited(monkeypatch):
+    captured = {}
+    monkeypatch.setattr(_isolation, "_MEM_LIMIT_APPLIED", False)
+    monkeypatch.setattr(
+        _isolation.resource,
+        "getrlimit",
+        lambda _w: (resource.RLIM_INFINITY, resource.RLIM_INFINITY),
+    )
+    monkeypatch.setattr(
+        _isolation.resource, "setrlimit", lambda _w, pair: captured.update(pair=pair)
+    )
+    # hard is unbounded -> soft is exactly the target, hard stays RLIM_INFINITY
+    _isolation._apply_mem_limit(1536)
+    soft, hard = captured["pair"]
+    assert soft == 1536 * 1024 * 1024
+    assert hard == resource.RLIM_INFINITY
+
+
 def test_apply_mem_limit_is_applied_once(monkeypatch):
     calls = []
     monkeypatch.setattr(_isolation, "_MEM_LIMIT_APPLIED", False)