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/dedup.py

@@ -113,6 +113,31 @@ _NOTE_OVERLAP_LIMIT = 3
 # gate was not part of the measurement, so it keeps the general bar.
 _COPY_BAND_TYPES = {"note"}
 
+# The constants above are the DEFAULTS; each is a setting (rule 25, #4385),
+# because how alike two distinct records get depends on how uniform a corpus
+# is, which nobody can know from here.
+GATE_THRESHOLD_KEYS = {
+    "general": "kb_gate_threshold",
+    SNIPPET_NOTE_TYPE: "kb_gate_threshold_snippet",
+    LESSON_NOTE_TYPE: "kb_gate_threshold_lesson",
+    "note_copy": "kb_gate_threshold_note_copy",
+    "note_overlap": "kb_gate_note_overlap_floor",
+}
+GATE_DEFAULT_THRESHOLDS = {
+    "general": _SEMANTIC_THRESHOLD,
+    SNIPPET_NOTE_TYPE: _SNIPPET_SEMANTIC_THRESHOLD,
+    LESSON_NOTE_TYPE: _LESSON_SEMANTIC_THRESHOLD,
+    "note_copy": _NOTE_COPY_THRESHOLD,
+    "note_overlap": _NOTE_OVERLAP_FLOOR,
+}
+# A BLOCK bar may not be set below this. The gate refuses the write, so a
+# mistyped 0.1 would refuse every create that shared a topic with anything —
+# the report's floors can go low because a report only proposes.
+_GATE_MIN_BLOCK = 0.80
+# The overlap floor only decides what is LISTED for the session to judge, so it
+# may go lower — but not so low that the three slots fill with noise.
+_GATE_MIN_OVERLAP = 0.70
+
 # The gate queries per CHUNK of the candidate (#280) — this caps how many
 # searches one save may cost. Eight chunks ≈ five thousand words of candidate;
 # a duplicate hiding past that is the duplicate report's job to find, not a
@@ -248,16 +273,43 @@ async def _find_snippet_by_structure(
 
 
 def _semantic_threshold(note_type: str) -> float:
-    """The semantic bar for this kind — a lookup, so the kinds that need a
-    different one are named in a single place rather than in a conditional
-    that grows a branch per kind."""
-    if note_type == SNIPPET_NOTE_TYPE:
-        return _SNIPPET_SEMANTIC_THRESHOLD
-    if note_type == LESSON_NOTE_TYPE:
-        return _LESSON_SEMANTIC_THRESHOLD
+    """The DEFAULT semantic bar for this kind — what `gate_bars` falls back on
+    when the user has not set one."""
+    return GATE_DEFAULT_THRESHOLDS[_gate_key(note_type)]
+
+
+def _gate_key(note_type: str) -> str:
+    if note_type in (SNIPPET_NOTE_TYPE, LESSON_NOTE_TYPE):
+        return note_type
     if note_type in _COPY_BAND_TYPES:
-        return _NOTE_COPY_THRESHOLD
-    return _SEMANTIC_THRESHOLD
+        return "note_copy"
+    return "general"
+
+
+async def _gate_setting(user_id: int, key: str, lo: float) -> float:
+    from scribe.services.settings import get_setting
+
+    default = GATE_DEFAULT_THRESHOLDS[key]
+    try:
+        value = float(await get_setting(user_id, GATE_THRESHOLD_KEYS[key], str(default)))
+    except Exception:
+        # Fail-open like the rest of the gate: an unreadable setting falls back
+        # to the measured default rather than blocking or waving through.
+        value = default
+    return min(1.0, max(lo, value))
+
+
+async def gate_bars(user_id: int, note_type: str) -> tuple[float, float]:
+    """(block_at, overlap_floor) for `note_type` on this user's install.
+
+    The overlap floor is only read for the copy-band kinds, and never sits
+    above the block bar — a floor over the bar would list nothing.
+    """
+    block_at = await _gate_setting(user_id, _gate_key(note_type), _GATE_MIN_BLOCK)
+    if note_type not in _COPY_BAND_TYPES:
+        return block_at, block_at
+    floor = await _gate_setting(user_id, "note_overlap", _GATE_MIN_OVERLAP)
+    return block_at, min(floor, block_at)
 
 
 @dataclass
@@ -355,7 +407,7 @@ async def find_duplicate_note(
         # under its name and embedded under `name — trigger`, so the query
         # document is built the way the corpus was, from `data`.
         doc_title = embeddings_svc.document_title(title, note_type, data, body)
-        block_at = _semantic_threshold(note_type)
+        block_at, overlap_floor = await gate_bars(user_id, note_type)
         collect = overlaps is not None and note_type in _COPY_BAND_TYPES
         near: dict[int, NoteOverlap] = {}
         for query in embeddings_svc.chunk_document(doc_title, body)[:_GATE_MAX_CHUNKS]:
@@ -369,7 +421,7 @@ async def find_duplicate_note(
                 user_id, query, project_id=project_id, is_task=is_task,
                 orphan_only=(project_id is None),
                 limit=3,
-                threshold=_NOTE_OVERLAP_FLOOR if collect else block_at,
+                threshold=overlap_floor if collect else block_at,
                 # Owner-only, deliberately: this gate BLOCKS a create and tells
                 # the caller to update the match instead. Matching someone
                 # else's record would refuse their write and point them at