stacks: remove 9 folders, update stack count in README

Removed: ollama (AI-related concerns), emby/qbittorrent/prowlarr
(media-download concerns), bookstack (being deprecated in the
homelab), and mariadb-latest/mysql-8/postgres-18-alpine/valkey-9.0-
alpine (build-only base-image factories - nothing connects to them,
they add no portfolio value on their own; real per-app DB usage,
e.g. Kodi's own mariadb service, already lives in that app's stack
folder and is unaffected).

Stack count updated 53 -> 44 in both README references.

stacks/platform/mariadb-latest/docker-compose.yaml

@@ -1,16 +0,0 @@
-services:
-  mariadb-latest:
-    container_name: mariadb-latest
-    image: mariadb:latest
-    restart: "no"
-    environment:
-      # No real data lives here - this is a thin self-hosted-registry mirror
-      # build target, not a running database. Random root password since
-      # the base image needs some root-auth setting to initialize at all.
-      - MYSQL_RANDOM_ROOT_PASSWORD=true
-    healthcheck:
-      test: ["CMD-SHELL", "mariadb-admin ping -h 127.0.0.1 -u root || exit 1"]
-      interval: 5s
-      timeout: 5s
-      retries: 10
-      start_period: 15s

stacks/platform/mysql-8/docker-compose.yaml

@@ -1,15 +0,0 @@
-services:
-  mysql-8:
-    container_name: mysql-8
-    image: mysql:8
-    restart: "no"
-    environment:
-      # No real data lives here - a thin self-hosted-registry mirror build
-      # target, not a running database.
-      - MYSQL_RANDOM_ROOT_PASSWORD=yes
-    healthcheck:
-      test: ["CMD-SHELL", "mysqladmin ping -h 127.0.0.1 || exit 1"]
-      interval: 5s
-      timeout: 5s
-      retries: 10
-      start_period: 20s

stacks/platform/postgres-18-alpine/docker-compose.yaml

@@ -1,17 +0,0 @@
-services:
-  postgres-18-alpine:
-    container_name: postgres-18-alpine
-    image: postgres:18-alpine
-    restart: "no"
-    environment:
-      # No real data lives here - a thin self-hosted-registry mirror build
-      # target so other stacks can pull the result. trust auth is fine
-      # since nothing reaches it but the health check below - Postgres
-      # refuses to start at all without some password/auth method set.
-      - POSTGRES_HOST_AUTH_METHOD=trust
-    healthcheck:
-      test: ["CMD-SHELL", "pg_isready -U postgres"]
-      interval: 5s
-      timeout: 5s
-      retries: 10
-      start_period: 10s

stacks/platform/valkey-9.0-alpine/docker-compose.yaml

@@ -1,11 +0,0 @@
-services:
-  valkey-9.0-alpine:
-    container_name: valkey-9.0-alpine
-    image: valkey/valkey:9.0-alpine
-    restart: "no"
-    healthcheck:
-      test: ["CMD", "valkey-cli", "ping"]
-      interval: 5s
-      timeout: 5s
-      retries: 10
-      start_period: 10s