If-Overwritten-Version header not enforced when accessed via TinyNode

## Summary
The `If-Overwritten-Version` header for optimistic locking is documented in the RERUM API but does not appear to be enforced when `/overwrite` requests come through TinyNode (tiny.rerum.io).

## Background
According to the [RERUM API documentation](https://store.rerum.io/v1/API.html#overwrite) and the server code in `controllers/overwrite.js`, the `If-Overwritten-Version` header should be checked against `__rerum.isOverwritten` to detect concurrent edit conflicts.

The server code shows this logic:
```javascript
const expectedVersion = req.get('If-Overwritten-Version') ?? req.body.__rerum?.isOverwritten
const currentVersionTS = originalObject.__rerum?.isOverwritten ?? ""
if (expectedVersion !== undefined && expectedVersion !== currentVersionTS) {
// Should return 409 Conflict
}
```

## The Problem
When accessing RERUM through TinyNode (`tiny.rerum.io/overwrite`), the `If-Overwritten-Version` header is sent correctly but the version check is not enforced - requests succeed even with version mismatches.

## Steps to Reproduce
1. Load an object via `store.rerum.io/v1/id/{id}` - note `__rerum.isOverwritten` (e.g., `2025-11-30T20:31:57.853`)
2. Send `/overwrite` to `tiny.rerum.io` - this updates `isOverwritten` to new value
3. Send another `/overwrite` to `tiny.rerum.io` with header `If-Overwritten-Version: 2025-11-30T20:31:57.853` (old value)
4. **Expected:** 409 Conflict
5. **Actual:** 200 OK, silent overwrite

## Questions
1. Does TinyNode pass through the `If-Overwritten-Version` header to RERUM?
2. Is the optimistic locking check implemented in TinyNode or expected to be handled by RERUM?
3. Is there a configuration needed to enable this feature?

## Related
- TinyNode issue: https://github.com/CenterForDigitalHumanities/TinyNode/issues/106
- RERUM API docs: https://store.rerum.io/v1/API.html#overwrite

Beginne mit controllers/overwrite.js und verfolge den /overwrite-Request-Pfad. Vergleiche ihn anschließend mit der /overwrite-Verarbeitung von TinyNode und dem zugehörigen TinyNode-Issue 106. Überprüfe, ob If-Overwritten-Version den RERUM-Server erreicht und ob nicht übereinstimmende Werte 409 Conflict statt 200 OK erzeugen. Abgeschlossen ist die Aufgabe, wenn das dokumentierte Verhalten der optimistischen Sperrung durchgesetzt wird oder die Integrations-/Konfigurationslücke eindeutig identifiziert ist.

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