Clean slate: 1h block sizes, remove backfill artifacts

- Changed all namespace block sizes to 1h (was 2h/12h/24h in manifests,
  30d+ in the live cluster due to backfill-era bufferPast hacks)
- Deleted entire backfill/ directory (scripts, pods, runbooks)
- Removed stale 05-m3coordinator.yaml (had backfill namespaces)
- Added 05-m3coordinator-deployment.yaml to kustomization
- Fixed init job health check (/health instead of /api/v1/services/m3db/health)
- Updated .env.example (removed Mimir credentials)
- Added 'Why Backfill Doesn't Work' section to README

kustomization.yaml

@@ -7,5 +7,5 @@ resources:
   - 02-etcd.yaml
   - 03-configmaps.yaml
   - 04-m3dbnode.yaml
-  - 05-m3coordinator.yaml
+  - 05-m3coordinator-deployment.yaml
   - 06-init-and-pdb.yaml