HydraIssues

## Problem

hydraheadwebstream and hydracluster currently hardcode a single global neckwebrtc controller URL. With the Brussels district now deployed at `hydraneckwebrtc-brussels.experiencenet.com`, sessions have no way to route to the correct regional controller.

The district field is already passed in session requests but is purely informational — it does not affect which controller handles the session.

## Proposed Solution

Add a district-to-controller mapping in both services:

### hydraheadwebstream
- Add `NeckWebRTCResolver` with `ForDistrict(district)` method
- Config gains `districts` map under `hydraneckwebrtc` section
- `handleWebSocket` resolves the correct client before creating a session
- Files: `internal/cli/serve.go`, `internal/client/neckwebrtc.go`, `internal/api/server.go`, `internal/api/handlers.go`

### hydracluster
- `buildNeckWebRTCWorkerConfig` uses `node.District` to pick the correct controller URL
- Config gains `districts` map under `hydraneckwebrtc` section
- Files: `internal/cli/cluster/serve.go`, `pkg/api/server.go`, `pkg/api/handlers_body.go`

## Config Example

```yaml
# hydraheadwebstream
hydraneckwebrtc:
default_url: "https://hydraneckwebrtc.experiencenet.com"
token: "..."
districts:
brussels:
url: "https://hydraneckwebrtc-brussels.experiencenet.com"

# hydracluster
hydraneckwebrtc:
controller_url: "https://hydraneckwebrtc.experiencenet.com"
controller_token: "..."
admin_token: "..."
districts:
brussels:
controller_url: "https://hydraneckwebrtc-brussels.experiencenet.com"
```

Backwards compatible — existing configs without `districts` continue to work unchanged.