add DigitalOcean pricing, startup validation, and demo update

- MetricsSource::DigitalOceanPricing variant: fetch public DO Gen-AI pricing, normalize as lowercase(creator)/model_id, cost = input + output per million
- cost_metrics endpoint format updated to { "model": { "input_per_million": X, "output_per_million": Y } }
- Startup errors: prefer:cheapest requires cost source, prefer:fastest requires prometheus
- Startup warning: models with no pricing/latency data ranked last
- One-per-type enforcement: digitalocean_pricing; error if cost_metrics + digitalocean_pricing both configured
- cost_snapshot() / latency_snapshot() on ModelMetricsService for startup checks
- Demo config updated to v0.4.0 top-level routing_preferences with cheapest + fastest policies
- docker-compose.yaml + prometheus.yaml + metrics_server.py for demo latency metrics
- Schema and docs updated

config/plano_config_schema.yaml

@@ -548,11 +548,24 @@ properties:
             refresh_interval:
               type: integer
               minimum: 1
+              description: "Refresh interval in seconds"
           required:
             - type
             - url
             - query
           additionalProperties: false
+        - type: object
+          properties:
+            type:
+              type: string
+              const: digitalocean_pricing
+            refresh_interval:
+              type: integer
+              minimum: 1
+              description: "Refresh interval in seconds"
+          required:
+            - type
+          additionalProperties: false
 
 additionalProperties: false
 required: