Add Docker health check support

Implements #406 by adding a --health CLI flag and HEALTHCHECK instruction
to the Dockerfile. This allows Docker and container orchestration platforms
to monitor container health automatically.

Changes:
- Added --health flag that validates configuration and plugin connectivity
- Implemented HealthCheck() method in pkg/poller/commands.go
- Updated Dockerfile with HEALTHCHECK instruction (30s interval, 10s timeout)
- Updated MANUAL.md with --health flag documentation
- Added health check documentation to Docker README
- Added comments to docker-compose examples about built-in health check

The health check:
- Validates configuration file is found and parseable
- Ensures at least one input and one enabled output are configured
- Performs basic validation on enabled outputs
- Returns exit code 0 (healthy) or 1 (unhealthy)
- Runs silently for Docker compatibility

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

init/synology-docker-compose/docker-compose.yml

@@ -36,6 +36,8 @@ services:
   unifi-poller:
     restart: always
     image: ghcr.io/unpoller/unpoller:${POLLER_TAG}
+    # Health check is built into the Docker image
+    # It validates configuration and checks plugin connectivity
     environment:
       - UP_INFLUXDB_USER=${INFLUXDB_ADMIN_USER}
       - UP_INFLUXDB_PASS=${INFLUXDB_ADMIN_PASSWORD}