Chunked request bodies silently dropped (breaks all logins via Cloudflare Tunnel)

### Describe the bug

POST request bodies sent with `Transfer-Encoding: chunked` (no `Content-Length`) are silently dropped before reaching the view layer. The request itself goes through — headers, method, and session are all intact — but the parsed body is empty.

The most visible symptom: **every login through a Cloudflare Tunnel fails** with `No identification data provided.` on the identification stage, because `cloudflared` always streams request bodies to the origin using chunked encoding. Clients that send `Content-Length` (browsers connecting directly, curl by default) are unaffected, which makes this look like a proxy or frontend problem when it is not.

### To reproduce

On any 2026.5.3 install, directly against the server container (no reverse proxy involved):

```bash
# Chunked: body is dropped -> "No identification data provided."
curl -s -X POST "http://localhost:9000/api/v3/flows/executor/default-authentication-flow/" \
-H "Accept: application/json" \
-H "Content-Type: application/json" \
-H "Transfer-Encoding: chunked" \
-d '{"component":"ak-stage-identification","uid_field":"someuser"}'

# Identical request with Content-Length: works (identification stage processes uid_field)
curl -s -X POST "http://localhost:9000/api/v3/flows/executor/default-authentication-flow/" \
-H "Accept: application/json" \
-H "Content-Type: application/json" \
-d '{"component":"ak-stage-identification","uid_field":"someuser"}'
```

The chunked variant returns the identification challenge with `"response_errors": {"non_field_errors": [{"string": "No identification data provided.", "code": "invalid"}]}` and no `invalid_login` event is logged (the serializer never sees `uid_field`). The Content-Length variant logs `invalid_login` / proceeds normally.

**Isolation:** the same pair of requests sent directly to the gunicorn unix socket inside the container (bypassing the embedded Go proxy entirely) reproduces the same behavior:

```bash
docker exec ak_server curl -s --unix-socket /dev/shm/authentik-core.sock \
-X POST "http://localhost/api/v3/flows/executor/default-authentication-flow/" \
-H "Content-Type: application/json" -H "Transfer-Encoding: chunked" \
-d '{"component":"ak-stage-identification","uid_field":"someuser"}'
```

So the body is lost in the gunicorn (25.3.0) / uvicorn (0.46.0) `DjangoUvicornWorker` / ASGI layer, not in the Go proxy.

### Expected behavior

Chunked request bodies are read and parsed the same as bodies with `Content-Length`. RFC 9112-compliant HTTP/1.1 clients (including `cloudflared`, which always uses chunked streaming to the origin) can log in.

### Logs

Nothing is logged for the failing requests beyond the normal access log line (status 200). No `invalid_login`, no error, no traceback — the body loss is silent.

### Version and Deployment

- authentik version: 2026.5.3 (`ghcr.io/goauthentik/server:2026.5.3`, gunicorn 25.3.0, uvicorn 0.46.0)
- Deployment: docker-compose
- Not yet retested on 2026.5.4/2026.5.5, but the changelogs for both show no changes to body/transport handling.

### Additional context

- Real-world impact: `Cloudflare edge -> cloudflared -> reverse proxy -> authentik` breaks all POSTs (logins, flow submissions) while GETs work, since cloudflared streams bodies chunked. Reproduced with cloudflared 2026.5.0 and 2026.7.2.
- Workaround: a buffering reverse proxy in front of authentik (e.g. Traefik `buffering` middleware) converts chunked uploads to `Content-Length` and restores logins.
- Possibly the actual root cause of #19552 — same error, and reporters there are also behind Cloudflare proxying.

Reproduce the paired curl requests against the server container and the gunicorn unix socket, then trace request-body handling through gunicorn 25.3.0, uvicorn 0.46.0, and DjangoUvicornWorker. Done means a request using Transfer-Encoding: chunked reaches the view layer with its JSON body parsed like the Content-Length variant, including through the authentication flow.

Written by the indexing model from the issue text.