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/docker-compose.yml

@@ -0,0 +1,49 @@
+services:
+  pihole:
+    container_name: pihole
+    # Root cause of the 2026-08 gravity.db corruption: default /dev/shm (64M)
+    # was too small for FTL's shared-memory metrics. 1024M has been stable.
+    shm_size: '1024M'
+    image: pihole/pihole:latest
+    hostname: pihole
+    entrypoint: ["/usr/local/bin/gravity-validate.sh"]
+    ports:
+      - "53:53/tcp"
+      - "53:53/udp"
+      - "10002:80/tcp"
+      - "10003:443/tcp"
+    environment:
+      TZ: 'America/Chicago'
+      FTLCONF_webserver_api_password: '${PIHOLE_WEB_PASSWORD}'
+      FTLCONF_dns_listeningMode: 'all'
+    volumes:
+      - './etc-pihole:/etc/pihole'
+      - './etc-dnsmasq.d:/etc/dnsmasq.d'
+      - './gravity-validate.sh:/usr/local/bin/gravity-validate.sh:ro'
+    cap_add:
+      - SYS_NICE
+    restart: always
+    healthcheck:
+      test: ["CMD-SHELL", "dig +short +norecurse @127.0.0.1 pi.hole >/dev/null 2>&1 && test -s /etc/pihole/gravity.db || exit 1"]
+      interval: 30s
+      timeout: 10s
+      retries: 3
+      start_period: 60s
+    labels:
+      autoheal: "true"
+    networks:
+      - default
+      - dnsnet
+  autoheal:
+    container_name: autoheal
+    image: willfarrell/autoheal:latest
+    environment:
+      AUTOHEAL_CONTAINER_LABEL: autoheal
+      AUTOHEAL_INTERVAL: 30
+      AUTOHEAL_START_PERIOD: 60
+    volumes:
+      - '/var/run/docker.sock:/var/run/docker.sock:ro'
+    restart: always
+networks:
+  dnsnet:
+    external: true