fix(ingest): address review round 3 + SonarCloud reliability gate

- tests: make the transient-backoff progression assertion load-independent by
  bracketing the get_retry_decision call with before/after timestamps instead of
  measuring against a second datetime.now() (no freezegun dependency).
- tests: use pytest.approx for the ingest-queue-depth gauge assertions —
  SonarCloud python:S1244 (float == ) was a MAJOR reliability finding that
  tripped the new_reliability_rating quality gate.
- processor: tighten the EscalateError lazy-bind comment (file processing already
  imports the document stack via get_registry; the gating only spares the
  delete / text-doc paths and module-load time).

Deck #323.

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

tests/unit/test_ingest_queue_depth_metric.py

@@ -5,6 +5,7 @@ procrastinate's ``list_queues_async``) must read 0, not its last non-zero value.
 """
 
 import pytest
+from pytest import approx
 
 from nextcloud_mcp_server.observability.metrics import update_ingest_queue_depth
 
@@ -14,19 +15,27 @@ _METRIC = "astrolabe_ingest_queue_depth"
 
 
 def test_drained_queue_zeroes_not_stale(metric_sample):
-    # ocr has a backlog this tick.
+    # ocr has a backlog this tick. (pytest.approx: the gauge sample is a float.)
     update_ingest_queue_depth({"ingest-ocr": {"todo": 4}})
-    assert metric_sample(_METRIC, {"queue": "ingest-ocr", "status": "todo"}) == 4.0
+    assert metric_sample(_METRIC, {"queue": "ingest-ocr", "status": "todo"}) == approx(
+        4
+    )
 
     # Next tick ocr has drained → procrastinate omits it from by_queue entirely.
     update_ingest_queue_depth({"ingest-fast": {"todo": 1}})
     # The gauge must read 0 for the drained queue, not the stale 4.
-    assert metric_sample(_METRIC, {"queue": "ingest-ocr", "status": "todo"}) == 0.0
-    assert metric_sample(_METRIC, {"queue": "ingest-fast", "status": "todo"}) == 1.0
+    assert metric_sample(_METRIC, {"queue": "ingest-ocr", "status": "todo"}) == approx(
+        0
+    )
+    assert metric_sample(_METRIC, {"queue": "ingest-fast", "status": "todo"}) == approx(
+        1
+    )
 
 
 def test_none_is_noop(metric_sample):
     update_ingest_queue_depth({"ingest-fast": {"doing": 2}})
     # Memory backend passes None → must not wipe the last published values.
     update_ingest_queue_depth(None)
-    assert metric_sample(_METRIC, {"queue": "ingest-fast", "status": "doing"}) == 2.0
+    assert metric_sample(
+        _METRIC, {"queue": "ingest-fast", "status": "doing"}
+    ) == approx(2)