Add a useDebouncedSave hook and delete Settings.stream

Two items from Stage 2 of `plan/17-refactor.md`.

`frontend/src/hooks/useDebouncedSave.js` replaces three copies of the same
debounce. `PlotPanel` and `ScenarioEditor` held identical per-key timer maps.
`ScriptEditor` held a single shared timer, so editing two fields inside the
same 600 ms window canceled the first save. The hook gives every key its own
timer, which fixes that.

`Settings.stream` was dead state. Nothing read it and every turn streams. This
removes the column, both schema fields, and adds migration 65 to drop it. It is
item S1 in `docs/self-review.md`.

Migration 65 needs a new guard. `_column_already_gone` is the counterpart to
`_column_already_there`: `create_all` builds the current schema, which is
already missing every dropped column, so a fixture that stamps an old version
and replays would fail on a column that is not there.

Verified: 549 backend tests pass, lint and build are clean, and migration 65
runs both ways, once against a database that still has the column and once
against one that does not.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014Dix4oGV3njgWRdu7P9t6r

frontend/src/pages/ScenarioEditor.jsx

@@ -1,7 +1,8 @@
-import { useEffect, useRef, useState } from 'react'
+import { useEffect, useState } from 'react'
 import { useNavigate, useParams } from 'react-router-dom'
 import { api } from '../api'
 import { Field, StoryCardRow, downloadJSON, pickJSONFile, useToast } from '../components'
+import { useDebouncedSave } from '../hooks/useDebouncedSave'
 import ArtPicker from '../ArtPicker'
 import SchemaEditor, { NpcEditor, addNpc } from '../SchemaEditor'
 
@@ -18,13 +19,7 @@ export default function ScenarioEditor() {
   const [parsedSchema, setParsedSchema] = useState(null)
   const [schemaView, setSchemaView] = useState('form') // 'form' | 'json'
   const toast = useToast()
-  // One timer per field/card: a single shared timer would cancel the pending
-  // save of whatever was edited previously within the debounce window.
-  const saveTimers = useRef(new Map())
-  const debounceSave = (key, fn) => {
-    clearTimeout(saveTimers.current.get(key))
-    saveTimers.current.set(key, setTimeout(fn, 600))
-  }
+  const debounceSave = useDebouncedSave()
 
   useEffect(() => {
     api.getScenario(id).then((s) => {