Major improvements: multi-stage Docker, security hardening, host metrics, tests

- Multi-stage Docker build: Dockerfile compiles frontend (Node.js) and
  backend (Gradle) inside Docker; no local Java/Node.js required
- Security hardening: replaced full host root mount with specific
  directory mounts (/proc, /sys, /etc, /var, /home, /opt, /tmp, /srv, /usr)
- Host metrics consistency: LinuxMemoryService and CpuDetailsService now
  read from /host/proc instead of container OSHI
- Concurrency: replaced synchronized with AtomicReference in 3 services
- Global error handling: added @RestControllerAdvice for JSON error responses
- Unit tests: added 32 tests for /proc parsers, CPU usage, process uptime
- Docker security: non-root user, read-only FS, no-new-privileges, healthcheck
- Debug logging: added log.debug() in silent catch blocks
- Documentation: updated README, INSTALLATION, DEVELOPMENT, CONFIGURATION,
  ARCHITECTURE, CHANGELOG

Co-authored-by: Junie <junie@jetbrains.com>

.idea/compiler.xml

@@ -1,5 +1,12 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <project version="4">
+  <component name="CompilerConfiguration">
+    <bytecodeTargetLevel>
+      <module name="backend" target="21" />
+      <module name="backend.main" target="21" />
+      <module name="backend.test" target="21" />
+    </bytecodeTargetLevel>
+  </component>
   <component name="JavacSettings">
     <option name="ADDITIONAL_OPTIONS_OVERRIDE">
       <module name="backend" options="-parameters" />