chore: initial import from KNEL/PFVCluster@041d311 [#769]

Split per O&M lane work order. Full history: KNEL/PFVCluster.
https://projects.knownelement.com/issues/769#note-4152

pihole/gravity-validate.sh

@@ -0,0 +1,28 @@
+#!/bin/bash
+# gravity-validate.sh — pre-start integrity check for Pi-hole's gravity.db
+#
+# Runs as the container entrypoint. If gravity.db is empty or has an invalid
+# SQLite header (the symptom of the /dev/shm corruption outage), move it aside
+# so Pi-hole regenerates a clean DB on start instead of crashing.
+set -e
+GRAVITY_DB="/etc/pihole/gravity.db"
+TIMESTAMP=$(date +%Y%m%d%H%M%S)
+
+if [ -f "$GRAVITY_DB" ]; then
+    if [ ! -s "$GRAVITY_DB" ]; then
+        echo "[gravity-validate] gravity.db is empty, moving aside"
+        mv "$GRAVITY_DB" "${GRAVITY_DB}.corrupt.${TIMESTAMP}"
+    else
+        HEADER=$(head -c 15 "$GRAVITY_DB" 2>/dev/null || true)
+        if [ "$HEADER" != "SQLite format 3" ]; then
+            echo "[gravity-validate] gravity.db invalid header, moving aside"
+            mv "$GRAVITY_DB" "${GRAVITY_DB}.corrupt.${TIMESTAMP}"
+        fi
+    fi
+fi
+# Keep only the 3 most recent corrupt backups (names carry a timestamp,
+# so lexical reverse-sort = newest-first).
+find /etc/pihole -maxdepth 1 -name 'gravity.db.corrupt.*' -print 2>/dev/null \
+  | sort -r | tail -n +4 | xargs -r rm -f
+echo "[gravity-validate] OK, starting Pi-hole"
+exec /usr/bin/start.sh "$@"