fix(netinfra): commit Pi-hole hardening config previously missing from repo

Commit 33b5c76 claimed to harden Pi-hole on both DNS nodes but only
modified markdown — the working docker-compose.yml, gravity-validate.sh,
healthcheck, and autoheal config were never written to the repo, leaving
the DNS hardening unreproducible from version control.

This commits the live, verified-working config from the boxes into
netinfra/pihole/:
- docker-compose.yml (shm_size 1024M root-cause fix, healthcheck, autoheal)
- gravity-validate.sh (pre-start SQLite header check, auto-quarantine corrupt DB)
- .env.example (web UI password templated; real .env gitignored)

Defends against the gravity.db / /dev/shm corruption production outage.
The live password is templated as ${PIHOLE_WEB_PASSWORD} so no secret
enters git.

[#376]

netinfra/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 "$@"