Fix Frigate parked car re-detection and enable writable config (#193)

## Summary
- Remove car-specific `max_frames: 150` which was causing a forget-and-re-detect loop on parked cars (every ~30 seconds at 5fps)
- Set `stationary.interval: 0` so Frigate never re-runs detection on stationary objects
- Replace read-only configmap subPath mount with initContainer + emptyDir, so Frigate UI changes (zones, masks) persist at runtime

## Context
Frigate was spamming notifications because `max_frames` for cars caused it to "forget" a parked car after 150 frames, then immediately re-detect it as a brand new object. The fix follows [Frigate's official parked cars guide](https://docs.frigate.video/guides/parked_cars/).

The writable config change also unblocks using `required_zones` for car alerts — zones can now be drawn in the Frigate UI and will survive until pod reschedule (at which point they should be baked into the configmap via IaC).

## Test plan
- [ ] Sync frigate app via ArgoCD and verify pod starts with initContainer
- [ ] Confirm parked cars no longer trigger repeated alerts
- [ ] Draw a zone/mask in Frigate UI, save, verify it persists after Frigate restart
- [ ] Set up `driveway_entrance` required zone for car alerts

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Reviewed-on: https://forge.ops.eblu.me/eblume/blumeops/pulls/193

docs/changelog.d/frigate-stationary-fix.bugfix.md

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+Fix Frigate repeatedly alerting on parked cars by removing per-object max_frames and setting stationary interval to 0. Make Frigate config writable so UI changes (zones, masks) persist within a pod lifecycle.