feat: Enhance gatekeeper resilience and host handling

This commit significantly improves the gatekeeper system's robustness, monitoring capabilities, and simplifies host header management for backend services.

Key changes include:

**Gatekeeper Health, Management & Resilience:**
- Implemented active health checking for individual gatekeeper containers within the `gatekeeper-manager` service.
    - The manager now periodically curls the `/metrics` endpoint of each gatekeeper container.
    - Reports health status to a new Gatus `services_gatekeeper` endpoint.
    - Automatically attempts to restart the gatekeeper stack if any gatekeeper instance is unhealthy or if the expected number of gatekeepers is not running.
- Refactored the `gatekeeper-manager` shell script for improved state management and signal handling:
    - Introduced `STARTED`, `RESTARTING`, `TERMINATING` state flags for more controlled operations.
    - Enhanced SIGTERM and SIGHUP handling to gracefully manage gatekeeper lifecycles.
    - Added `apk add curl` to ensure `curl` is available in the manager container.
- Renamed the gatekeeper Docker Compose template from `docker-compose_gatekeeper.template.yml` to `gatekeepers.template.yml` and its output to `gatekeepers.yml`.
- Updated `dockergen-gatekeeper` to watch the new template file and notify the correct `gatekeeper-manager` service instance (e.g., `pkmntrade-club-gatekeeper-manager-1`).
- Discover services that should be protected by looking for a `gatekeeper=true` label.

**Host Header Management & `ALLOWED_HOSTS` Simplification:**
- HAProxy configuration (`haproxy.cfg`) now consistently sets the `Host` HTTP header for requests to all backend services (e.g., `pkmntrade.club`, `staging.pkmntrade.club`). This centralizes and standardizes host information.
- Consequently, explicit `ALLOWED_HOSTS` environment variables have been removed from the `web` and `celery` service definitions in `docker-compose_web.yml` and `docker-compose_staging.yml`. Backend Django applications should now rely on the `Host` header set by HAProxy for request validation.
- The `gatekeepers.template.yml` now defines a `TARGET_HOST` environment variable for proxied services (e.g., `web`, `web-staging`). This aligns with the ALLOWED_HOSTS on the target to ensure requests aren't blocked.

**Gatus Monitoring & Configuration Updates:**
- In Gatus configuration (`gatus/config.template.yaml`):
    - The "Redis" external service endpoint has been renamed to "Cache" for better clarity and to fit the theme of simple names.
    - A new external service endpoint "Gatekeeper" has been added to monitor the overall health reported by the `gatekeeper-manager`.
    - Health checks for "Web Worker" endpoints (both main and staging) now include the appropriate `Host` header (e.g., `Host: pkmntrade.club`) to ensure accurate health assessments by Django.
- In `docker-compose_core.yml`, the `curl` commands used by `db-redis-healthcheck` for database and cache health now append `|| true`. This prevents the script from exiting on a curl error (e.g., timeout, connection refused), ensuring that the failure is still reported to Gatus via the `success=false` parameter rather than the script terminating prematurely.

These changes collectively make the gatekeeper system more fault-tolerant, provide better visibility into its status, and streamline the configuration of backend applications by standardizing how they receive host information.

server/docker-compose_gatekeeper.template.yml

@@ -1,37 +0,0 @@
-services:
-  {{ $all_containers := whereLabelValueMatches . "enable_gatekeeper" "true" }}
-  {{ $all_containers = sortObjectsByKeysAsc $all_containers "Name" }}
-
-  {{ range $container := $all_containers }}
-  {{ $serviceLabel := index $container.Labels "com.docker.compose.service" }}
-  {{ $containerNumber := index $container.Labels "com.docker.compose.container-number" }}
-  {{ $port := "" }}
-  {{ if eq $serviceLabel "web" }}
-    {{ $port = ":8000" }}
-  {{ end }}
-  {{ if eq $serviceLabel "web-staging" }}
-    {{ $port = ":8000" }}
-  {{ end }}
-  {{ if eq $serviceLabel "feedback" }}
-    {{ $port = ":3000" }}
-  {{ end }}
-  {{ if eq $serviceLabel "health" }}
-    {{ $port = ":8080" }}
-  {{ end }}
-  gatekeeper-{{ $serviceLabel }}-{{ $containerNumber }}:
-    image: ghcr.io/techarohq/anubis:latest
-    container_name: pkmntrade-club-gatekeeper-{{ $serviceLabel }}-{{ $containerNumber }}
-    env_file:
-      - .env
-    environment:
-      - TARGET=http://{{ $container.Name }}{{ $port }}
-    networks:
-      default:
-        aliases:
-          - pkmntrade-club-gatekeeper-{{ $serviceLabel }}
-          - gatekeeper-{{ $serviceLabel }}
-  {{ end }}
-networks:
-  default:
-    name: pkmntrade-club_default
-    external: true