Bug: TemplateData pre-save hook uses next callback (Mongoose 9.x incompatible)

## Bug: TemplateData pre-save hook uses `next` callback (Mongoose 9.x incompatible)

**File:** `server/models/TemplateData.js:52-55`

### Problem
The pre-save hook uses the old Mongoose pattern with `next` callback:
```js
TemplateDataSchema.pre('save', function (next) {
  this.updatedAt = new Date();
  next();
});
```

In Mongoose 9.x, the `next` parameter is no longer passed to `pre('save')` hooks. This causes `TypeError: next is not a function` on every `TemplateData.create()` or `.save()` call.

### Fix
Remove the `next` callback — Mongoose 9.x automatically calls `next()` after the hook function returns:
```js
TemplateDataSchema.pre('save', function () {
  this.updatedAt = new Date();
});
```

### Severity
**Critical** — Every template creation/save throws an error. The sync endpoint and any future template writes are broken.

### Status
Fix already exists in PR #29 (Phase 4). This issue tracks it for visibility.

### Phase
Introduced in Phase 1 (PR #26, merged).


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Bug: TemplateData pre-save hook uses next callback (Mongoose 9.x incompatible) #33