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/ScriptEditor.jsx

@@ -1,10 +1,11 @@
-import { useEffect, useRef, useState } from 'react'
+import { useEffect, useState } from 'react'
 import { useNavigate, useParams } from 'react-router-dom'
 import CodeMirror from '@uiw/react-codemirror'
 import { javascript } from '@codemirror/lang-javascript'
 import { oneDark } from '@codemirror/theme-one-dark'
 import { api } from '../api'
 import { Field, downloadJSON } from '../components'
+import { useDebouncedSave } from '../hooks/useDebouncedSave'
 
 const SLOTS = [
   { key: 'library_js', label: 'Library', hint: 'Shared code prepended to all three hooks.' },
@@ -19,7 +20,7 @@ export default function ScriptEditor() {
   const [script, setScript] = useState(null)
   const [slot, setSlot] = useState('input_js')
   const [status, setStatus] = useState('')
-  const saveTimer = useRef(null)
+  const debounceSave = useDebouncedSave()
 
   // Test-run state
   const [testHook, setTestHook] = useState('input')
@@ -33,12 +34,11 @@ export default function ScriptEditor() {
 
   const setField = (field, value) => {
     setScript((prev) => ({ ...prev, [field]: value }))
-    clearTimeout(saveTimer.current)
-    saveTimer.current = setTimeout(async () => {
+    debounceSave(field, async () => {
       await api.updateScript(id, { [field]: value })
       setStatus('Saved')
       setTimeout(() => setStatus(''), 1500)
-    }, 600)
+    })
   }
 
   const runTest = async () => {