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]

docs/docmap.md

@@ -3,7 +3,7 @@
 > **Index of all documentation in this repo.** Agents must update this file
 > whenever a doc is added, removed, or substantively changed.
 
-> **Last updated:** 2026-08-05 (full read-only ground-truth audit: network+SNMP+compute+k8s+netinfra)
+> **Last updated:** 2026-08-06 (Pi-hole hardening config committed to netinfra/pihole/ — [#376])
 
 ## Kubernetes Architecture ([`k8s/`](k8s/))
 
@@ -61,6 +61,7 @@ Server provisioning, security hardening, DNS/NTP configuration.
 | [`../netinfra/dns-cluster-setup/README.md`](../netinfra/dns-cluster-setup/README.md) | Technitium DNS cluster setup: export, deploy, cluster, verify | 2026-07-28 |
 | [`../tests/README.md`](../tests/README.md) | Test suite documentation: unit, security, validation tests | 2026-07-28 |
 | [`../netinfra/pfv-netinfra-setup.md`](../netinfra/pfv-netinfra-setup.md) | pfv-netinfra-01/02 initial setup guide | 2026-07-27 |
+| [`../netinfra/pihole/README.md`](../netinfra/pihole/README.md) | Pi-hole recursive DNS on netinfra-01/02: shm_size fix, gravity-validate.sh, healthcheck, autoheal (defense-in-depth vs gravity.db corruption) | 2026-08-06 |
 | [`../netinfra/pfv-netboot-setup.md`](../netinfra/pfv-netboot-setup.md) | pfv-netboot reference node setup | 2026-07-27 |
 
 ## Archive ([`archive/`](archive/))