fix(review): type timeout as float; document worker reuse + identity check

Address PR #852 round 3 (all 🟡, no blockers):

- config: DOCUMENT_PARSE_TIMEOUT_SECONDS is now float (default 120.0) so a
  fractional value is honoured rather than silently stored in an int field;
  matches anyio.move_on_after's float seconds.
- _isolation: comment that a clean rlimit MemoryError leaves the worker alive
  in anyio's pool (vs the SIGKILL/BrokenWorkerProcess path that respawns) --
  acceptable since RLIMIT_AS caps virtual address space, not RSS.
- processor: note the `if indexed is False` is a deliberate identity check --
  a successful index (incl. dedup hit) returns None and must not be mistaken
  for a parse failure.

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

nextcloud_mcp_server/document_processors/_isolation.py

@@ -115,6 +115,11 @@ async def run_isolated_pdf_parse(
                 cancellable=True,
             )
         except MemoryError as e:
+            # A clean rlimit breach: the worker raised MemoryError and stays
+            # ALIVE in anyio's pool (unlike the BrokenWorkerProcess/SIGKILL path,
+            # which spawns a fresh worker). Its heap may be slightly fragmented
+            # for the next document. Acceptable: RLIMIT_AS caps virtual address
+            # space (not RSS), so practical fragmentation risk is low.
             raise PdfParseFailed("oom", str(e)) from e
         except BrokenWorkerProcess as e:
             # Worker died without a clean exception (e.g. SIGKILL from the OS OOM