Add websocket_connected metric and configurable prometheus_job_id

[vineeshah]

Resolves #44 and #45.

#44 - new websocket_connected gauge (1 connected / 0 not), set on smee connect and reset in the finally so every disconnect path reports 0.

#45 - parse optional prometheus_job_id from config.yml so multiple sce-cicd instances stop overwriting each other's pushgateway metrics. Falls back to "sce-cicd" when unset.

Tested locally against smee2 + one.sce: connect→1, kill smee2→0, custom job id lands as its own group, mock push pushes last_push_timestamp{repo}. Screenshot of test added below.

[adarshm11]

looks good but can we split into two PRs, one resolving each issue

[vineeshah]

Split into two focused PRs, one per issue: #47 (#44 websocket_connected metric) and #48 (#45 configurable prometheus_job_id). Closing this in favor of those.