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

06-init-and-pdb.yaml

@@ -63,7 +63,7 @@ spec:
               COORD="http://m3coordinator.m3db.svc.cluster.local:7201"
 
               echo "=== Waiting for coordinator to be healthy ==="
-              until curl -sf "${COORD}/api/v1/services/m3db/health"; do
+              until curl -sf "${COORD}/health"; do
                 echo "Coordinator not ready yet, retrying in 5s..."
                 sleep 5
               done
@@ -121,13 +121,13 @@ spec:
                     "repairEnabled": false,
                     "retentionOptions": {
                       "retentionPeriodDuration": "48h",
-                      "blockSizeDuration": "2h",
+                      "blockSizeDuration": "1h",
                       "bufferFutureDuration": "10m",
                       "bufferPastDuration": "10m"
                     },
                     "indexOptions": {
                       "enabled": true,
-                      "blockSizeDuration": "2h"
+                      "blockSizeDuration": "1h"
                     }
                   }
                 }'
@@ -146,13 +146,13 @@ spec:
                     "snapshotEnabled": true,
                     "retentionOptions": {
                       "retentionPeriodDuration": "720h",
-                      "blockSizeDuration": "12h",
+                      "blockSizeDuration": "1h",
                       "bufferFutureDuration": "10m",
                       "bufferPastDuration": "10m"
                     },
                     "indexOptions": {
                       "enabled": true,
-                      "blockSizeDuration": "12h"
+                      "blockSizeDuration": "1h"
                     },
                     "aggregationOptions": {
                       "aggregations": [
@@ -181,13 +181,13 @@ spec:
                     "snapshotEnabled": true,
                     "retentionOptions": {
                       "retentionPeriodDuration": "8760h",
-                      "blockSizeDuration": "24h",
+                      "blockSizeDuration": "1h",
                       "bufferFutureDuration": "10m",
                       "bufferPastDuration": "10m"
                     },
                     "indexOptions": {
                       "enabled": true,
-                      "blockSizeDuration": "24h"
+                      "blockSizeDuration": "1h"
                     },
                     "aggregationOptions": {
                       "aggregations": [