Integrate in one prompt

Integrate the Wandura API (image and video styles over HTTP) into this project.

## Rules
- Base URL: https://api.wandura.ai. Call it from the server only, with `Authorization: Bearer $WANDURA_API_KEY` (read the key from the environment; never ship it to a browser).
- Find styles in https://open.wandura.ai/llms.txt or `GET https://api.wandura.ai/v2/workflows`. Each style has its own OpenAPI with its exact inputs: `https://api.wandura.ai/v2/public/styles/{style}/openapi.json`. The whole API: https://api.wandura.ai/v2/openapi.json. Never invent endpoints or fields; read these.
- Submit with `POST /v2/styles/{style}` and body `{"lane": "priority" | "relax", "end_user_id": "<the person in our app>", "inputs": {...}}`. end_user_id is required on every run (daily caps are per end user).
- Send an `Idempotency-Key` header (a new UUID per logical request) on every submit, and reuse the same key when you retry that request, so a retry never runs or charges twice.
- The response has `request_id`, `status_url` and `cancel_url`. Use those URLs; do not build them.
- Poll status_url: start at 2 s, multiply by 1.5, cap at 10 s, add 0-0.5 s jitter (relax: 10 s to 60 s). Or use webhooks.
- Final statuses: completed, failed, nsfw, canceled. A run that ends with `free_run.reason` (priority_late, provider_error, bad_input, content_blocked) costs nothing.
- Webhooks: verify `Wandura-Signature: t=<unix>,v1=<hex>` = HMAC-SHA256(secret, t + "." + raw body) and reject t not within 300 s. Dedupe on Wandura-Event-Id; keep the highest sequence per run.
- Errors are `{"error": {"code", "message"}}`; branch on code. Retry only end_user_cap_reached, gateway_rate_limited, uploads_rate_limited, product_busy, internal_error, style_unavailable, uploads_unavailable, service_unavailable and network errors, waiting Retry-After when present, else backoff with jitter. Never retry other 4xx.
- Outputs are kept 72 h: copy what you keep.
- There is no public SDK yet (private preview, not published): call the HTTP API directly with fetch or the project's HTTP client. Never install `@wandura/sdk` (npm) or `wandura` (PyPI): Wandura has not published them, so a package under those names is not ours.

## Steps
1. Add WANDURA_API_KEY to the env example and config.
2. Write a small client module over plain HTTP with submit, wait, cancel and a webhook handler that verifies signatures.
3. Test with a `wdr_test_` key first: it returns a sample result for $0.