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/vector/processor.py

@@ -244,6 +244,9 @@ async def process_document(
                     # document_parse_total{error}) and the placeholder marked
                     # "failed". It is not an indexing event and not retryable, so
                     # don't count it as a successful upsert/indexed document.
+                    # Identity check, not `if not indexed`: a successful index
+                    # (including a dedup hit) returns None, which must NOT be
+                    # treated as a parse failure.
                     if indexed is False:
                         return