Implement the following plan:

# Plan: Simplify Mobile Writing API

## Context

The mobile app has two write endpoints (v1/v2) doing overlapping things, no mobile checkpoint support, no way to poll for downstream status, and a critical bug where mobile writes never create the anky DB record before spawning the image pipeline (so `UPDATE ankys` silently fails). We're collapsing this into one clean flow.

## Changes

### 1. Fix the anky creation bug (critical)
### 2. Unify to one write endpoint: `POST /swift/v2/write`
### 3. Add status endpoint: `GET /swift/v2/writing/{sessionId}/status`
### 4. Route changes in `src/routes/mod.rs`
### 5. Deprecate v1/write behavior