Add OpenAPI/Swagger spec for dashboard API

[dcondrey]

Difficulty: Moderate

Document the dashboard API with an OpenAPI 3.0 spec so third-party tools can integrate.

Endpoints to document:

• `GET /api/v1/locations?days=N`
• `GET /api/v1/stats`
• `GET /api/v1/health`
• `GET /api/v1/poll-status`
• `GET /api/v1/export?format=json|csv|geojson`
• `POST /api/v1/self-location`

Deliverables:

• `openapi.yaml` in repo root
• Optional: serve at `/api/docs` using flask-swagger-ui or similar