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/gatus/config.template.yaml

@@ -13,7 +13,12 @@ external-endpoints:
     token: "${GATUS_TOKEN}"
     alerts:
       - type: email
-  - name: Redis
+  - name: Cache
+    group: Services
+    token: "${GATUS_TOKEN}"
+    alerts:
+      - type: email
+  - name: Gatekeeper
     group: Services
     token: "${GATUS_TOKEN}"
     alerts:
@@ -68,8 +73,8 @@ endpoints:
       - "[DNS_RCODE] == NOERROR"
     alerts:
       - type: email
-  - name: HAProxy
-    group: Load Balancer
+  - name: Load Balancer
+    group: Services
     url: "http://loba/"
     interval: 60s
     conditions:
@@ -78,8 +83,8 @@ endpoints:
     alerts:
       - type: email
   - name: Feedback
-    group: Services
-    url: "http://feedback:3000/"
+    group: Main
+    url: "http://pkmntrade-club-feedback-1:3000/"
     interval: 60s
     conditions:
       - "[STATUS] == 200"
@@ -107,6 +112,8 @@ endpoints:
   - name: "Web Worker {{ $containerNumber }}"
     group: Main
     url: "http://{{ $container.Name }}:8000/health/"
+    headers:
+      Host: "pkmntrade.club"
     interval: 60s
     conditions:
       - "[STATUS] == 200"
@@ -120,6 +127,8 @@ endpoints:
   - name: "Web Worker {{ $containerNumber }}"
     group: Staging
     url: "http://{{ $container.Name }}:8000/health/"
+    headers:
+      Host: "staging.pkmntrade.club"
     interval: 60s
     conditions:
       - "[STATUS] == 200"