feat: real alert delivery channels (Slack/Discord/PagerDuty/webhook)

[ryzrr]

Summary

• Backs the Alerts "Channels" tab with real per-project destinations: new alert_channels table (migration 006), CRUD + test-delivery endpoints (/v1/channels...), and core/notify.deliver which POSTs to Slack/Discord/PagerDuty/generic webhooks.
• Adds worker/alerts.py, an evaluation loop (every 30s) that checks each enabled alert rule against live metrics, flips firing/resolved state, writes alert_history, and delivers notifications through the configured channel.
• Fixes anomaly detection so it only fires on metric increases — a drop in error rate or p99 latency is an improvement, not an incident, and previously could still page someone.
• Frontend: Alerts page now manages real channels (add/edit/test/delete) instead of mock state; Alerts/Endpoints/Traces breadcrumbs show the active project's real name instead of a hardcoded placeholder; alert rules/history poll every 15s so firing state updates live.

Test plan

• Run migration 006 and confirm alert_channels table + RLS policy are created
• Create a Slack/Discord/webhook channel from the Alerts → Channels tab and send a test message
• Create an alert rule pointed at that channel, breach the threshold, and confirm delivery + alert_history row
• Confirm resolving the breach flips the rule back to ok and closes the history row
• Verify a metric decrease (e.g. error rate dropping) no longer triggers an anomaly alert