Add Prometheus metrics, JSON logs and provider health checks

- GET /metrics (bearer server.metrics_token): build info, users, sessions,
  pending requests, per-model request/token/cost counters, provider_up
- CODECRAFT_LOG_FORMAT=json for one JSON object per log line
- Providers are probed every 5 minutes via their model list; status shown
  in the provider settings with a manual re-check

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

server/Cargo.toml

@@ -37,7 +37,7 @@ toml = "1"
 toml_edit = "0.25"
 tower-http = { version = "0.7", features = ["fs", "cors", "trace"] }
 tracing = "0.1"
-tracing-subscriber = { version = "0.3", features = ["env-filter"] }
+tracing-subscriber = { version = "0.3", features = ["env-filter", "json"] }
 uuid = { version = "1", features = ["v4", "v7"] }
 p256 = { version = "0.14", features = ["ecdh", "ecdsa"] }
 hkdf = "0.13"