feat: drawer bulk significance re-rate per chat (T110.4)

The drawer's Significance review panel previously only supported
per-memory edits. Adds a bulk control: pick ``level_from`` and
``level_to``, and every memory in the chat at ``level_from`` is moved
to ``level_to``.

Implementation emits one ``manual_edit`` event per matching memory
(not a single bulk event) so the §6.4 per-row audit trail stays
intact — each affected memory carries its own ``prior_value -> new_value``
snapshot, so an inverse edit can restore an individual row without
needing to inspect a bulk payload's member list. Reuses the existing
``memory_significance`` ``manual_edit`` projector branch (T25), so no
state-layer changes are required.

The route rejects no-op submissions (``level_from == level_to``) with
400 to avoid padding the event log with empty edits, and clamps both
levels to 0..3 (matching ``edit_memory_significance``).

UI: a small ``<details>`` block in the Significance review section
with two number inputs and a submit button.

Test: tests/test_drawer_phase4.py::test_bulk_significance_re_rate_emits_manual_edit_per_memory.

chat/templates/_drawer.html

@@ -547,6 +547,25 @@
         </ul>
       </details>
     {% endif %}
+    {# T110.4: bulk significance re-rate. Move every memory in this chat
+       at level_from to level_to with one manual_edit event per row, so
+       the audit trail stays per-memory. #}
+    <details class="bulk-significance">
+      <summary>Bulk re-rate significance</summary>
+      <form class="inline-edit"
+            hx-post="/chats/{{ chat.id }}/drawer/memory/significance/bulk"
+            hx-target="#drawer" hx-swap="innerHTML">
+        <label>
+          From:
+          <input type="number" name="level_from" min="0" max="3" value="0" required>
+        </label>
+        <label>
+          To:
+          <input type="number" name="level_to" min="0" max="3" value="1" required>
+        </label>
+        <button type="submit">Re-rate all</button>
+      </form>
+    </details>
   </section>
 
   <section class="drawer-section">

chat/web/drawer.py

@@ -411,6 +411,64 @@ async def edit_memory_significance(
     return await drawer(chat_id, request, conn)
 
 
+@router.post(
+    "/chats/{chat_id}/drawer/memory/significance/bulk",
+    response_class=HTMLResponse,
+)
+async def bulk_re_rate_significance(
+    chat_id: str,
+    request: Request,
+    level_from: int = Form(...),
+    level_to: int = Form(...),
+    conn=Depends(get_conn),
+):
+    """T110.4: bulk re-rate every memory in this chat at ``level_from``
+    to ``level_to``.
+
+    Fans out into one ``manual_edit`` event per matching memory rather
+    than a single bulk event so the §6.4 audit trail stays per-row —
+    each affected memory carries its own ``prior_value -> new_value``
+    snapshot, so an inverse edit can restore an individual row without
+    needing to inspect a bulk payload's member list. The drawer's
+    significance-distribution panel surfaces the new buckets on the
+    refreshed partial.
+
+    Both levels are clamped to 0..3 (matching ``edit_memory_significance``)
+    and a no-op (``level_from == level_to``) is rejected with 400 so a
+    misclick can't pad the event log with empty edits.
+    """
+    chat = get_chat(conn, chat_id)
+    if chat is None:
+        raise HTTPException(status_code=404, detail=f"chat not found: {chat_id}")
+
+    lf = max(0, min(3, int(level_from)))
+    lt = max(0, min(3, int(level_to)))
+    if lf == lt:
+        raise HTTPException(
+            status_code=400,
+            detail=f"level_from and level_to must differ (both = {lf})",
+        )
+
+    rows = conn.execute(
+        "SELECT id FROM memories WHERE chat_id = ? AND significance = ? "
+        "ORDER BY id ASC",
+        (chat_id, lf),
+    ).fetchall()
+    for row in rows:
+        memory_id = int(row[0])
+        append_and_apply(
+            conn,
+            kind="manual_edit",
+            payload={
+                "target_kind": "memory_significance",
+                "target_id": memory_id,
+                "prior_value": lf,
+                "new_value": lt,
+            },
+        )
+    return await drawer(chat_id, request, conn)
+
+
 @router.post(
     "/chats/{chat_id}/drawer/memory/{memory_id}/pin",
     response_class=HTMLResponse,