I need you to implement three things:

## 1. x402 Payment for Prompt Creation via API

Add an API endpoint that lets agents create writing prompts and pay via x402 (Coinbase payment protocol).

`POST /api/v1/prompt/create`

Request:
{
  "prompt_text": "tell me who you are",
  "created_by": "agent_name"  // optional
}

Headers:
- x402 payment headers OR X-API-Key with balance

Price: $0.25 per prompt (same as generation cost)

Response (the full JSON object the agent needs):
{
  "prompt_id": "uuid",
  "prompt_text": "tell me who you are",
  "url": "https://anky.app/prompt/{uuid}",
  "image_url": "https://anky.app/data/images/prompt_{uuid}.png",
  "created_at": "ISO-8601",
  "writers_count": 0
}

The prompt should generate an OG image automatically (like it already does for existing prompts). The returned JSON gives the agent everything it needs to compose a social media post with the prompt embedded.

## 2. Writers Count per Prompt

Track how many writing sessions have been submitted for each prompt. Add a `writers_count` column to the prompts table (or however prompts are stored).

Increment it each time someone submits writing via `POST /api/v1/prompt/{id}/write`.

Expose it in:
- `GET /api/v1/prompt/{id}` → returns prompt details including `writers_count`
- `GET /api/v1/prompts` → lists all prompts with their `writers_count`, sorted by most recent. Support `?sort=popular` for sorting by writers_count desc.

## 3. Prompt Detail API

`GET /api/v1/prompt/{id}`

Response:
{
  "prompt_id": "uuid",
  "prompt_text": "tell me who you are",
  "url": "https://anky.app/prompt/{uuid}",
  "image_url": "https://anky.app/data/images/prompt_{uuid}.png",
  "writers_count": 42,
  "created_at": "ISO-8601",
  "created_by": "Anky"
}

This is so an AI agent can:
1. Create a prompt (paying with x402 or API key)
2. Get back the full JSON with image URL
3. Post it on Twitter/social with the OG image embedded
4. Later check how many people wrote for that prompt

Keep it consistent with the existing API style in the codebase. The x402 payment flow should match how /api/v1/generate already handles payments.