feat(dedup): the create gate's similarity bars are settings (#4385, rule 25)

gate_bars(user_id, note_type) resolves the block bar and, for notes and
tasks, the overlap floor from kb_gate_* settings, with the old constants
as defaults. Fail-open on an unreadable value; a block bar clamps at 0.80
and the overlap floor at 0.70 and never above the bar. Five fields in
Settings beside the duplicate-report floors.

CLAIM_LEASE stays a constant, with the reason written at it: a per-user
lease would make one shared task live to one reader and dead to another.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/scribe/services/task_claims.py

@@ -37,6 +37,12 @@ if TYPE_CHECKING:
 # compaction, a resume or a long read does not kill it; short enough that a
 # session gone overnight reads as gone. The cost either way is stated rather
 # than hidden: readers show the age beside `live`, never the boolean alone.
+#
+# NOT A SETTING, deliberately (#4385 weighed it). Settings are per user, and a
+# claim is read by everyone who can see the task: a per-user lease would make
+# one shared task read as live to one collaborator and dead to another, and
+# "is anyone on this?" only means something if every reader gets the same
+# answer. It is also read synchronously in `to_dict`, which has no user to ask.
 CLAIM_LEASE = timedelta(hours=2)