Added Sentry error reporting and Oban monitoring

Hive now reports errors to Sentry. Set SENTRY_DSN to enable it, and optionally SENTRY_ENVIRONMENT and SENTRY_RELEASE to tag events. When Sentry is configured, Hive captures Phoenix requests, LiveView errors, Logger errors, and Oban job failures, including scheduled-job check-ins. Retryable Oban failures are reported by default; set SENTRY_OBAN_REPORT_RETRIES=false to only report exhausted jobs. The Helm chart includes defaults for the Oban settings, and the deployment tags releases so errors can be tied to a specific version. Deployments without SENTRY_DSN keep the previous behavior, so self-hosting without Sentry is unaffected.