Sentry error reporting integration

Hive now integrates Sentry for error reporting so operators can receive application errors with environment and release tags. Self-hosters can enable it by setting SENTRY_DSN along with optional SENTRY_ENVIRONMENT and SENTRY_RELEASE. When configured, Hive reports Phoenix request errors, LiveView errors, Logger errors, Oban job failures, and scheduled job check-ins. Oban retries are reported by default, but operators can set SENTRY_OBAN_REPORT_RETRIES=false to report only jobs that have exhausted all attempts. Deployments without SENTRY_DSN remain unchanged.