feat(core): implement modular OOP engine following SOLID and DRY principles

- models: DomainConfig, ServiceConfig, DiagnosticReport (single source of truth)

- interfaces: DIP & ISP contracts for detector, generator, executor, doctor, and migrator

- detector: Smart auto-detection for modular (config/wsgi.py), traditional, and flat layouts

- service_generator: Systemd service generation with --umask 007 and security sandboxing

- executor: Linux command execution with dry-run support

- nginx_manager: HestiaCP template installation and validation

- migrator: Automated migration and repair of existing deployments

- doctor: Comprehensive post-deployment health check engine

core/nginx_manager.py

@@ -0,0 +1,59 @@
+# -*- coding: utf-8 -*-
+"""
+NGINX template manager for HestiaCP.
+
+**Single Responsibility:** Install, validate, and manage the custom NGINX
+proxy templates that HestiaCP uses to route traffic to the Gunicorn socket.
+"""
+
+from __future__ import annotations
+
+from pathlib import Path
+
+from core.interfaces import INginxManager, ISystemExecutor
+from core.models import HESTIA_TEMPLATE_DIR, TEMPLATE_NAME, DomainConfig
+
+
+class HestiaNginxManager(INginxManager):
+    """Manages NGINX template deployment for HestiaCP."""
+
+    def __init__(self, executor: ISystemExecutor) -> None:
+        self._executor = executor
+
+    @property
+    def tpl_dest(self) -> Path:
+        return HESTIA_TEMPLATE_DIR / f"{TEMPLATE_NAME}.tpl"
+
+    @property
+    def stpl_dest(self) -> Path:
+        return HESTIA_TEMPLATE_DIR / f"{TEMPLATE_NAME}.stpl"
+
+    def install_templates(self, tpl_source: Path, stpl_source: Path) -> None:
+        """Copy templates to the HestiaCP template directory and set permissions."""
+        for src, dst in [(tpl_source, self.tpl_dest), (stpl_source, self.stpl_dest)]:
+            if not self._executor.file_exists(src):
+                raise FileNotFoundError(f"Template source not found: {src}")
+
+            content = src.read_text(encoding="utf-8")
+            self._executor.write_file(dst, content, mode=0o644)
+
+        # Set ownership to root:root
+        self._executor.run(
+            f"chown root:root {self.tpl_dest} {self.stpl_dest}"
+        )
+
+    def rebuild_domain(self, domain_config: DomainConfig) -> None:
+        """Force-rebuild the NGINX configuration for a domain and restart."""
+        dc = domain_config
+        self._executor.run(
+            f"v-rebuild-web-domain {dc.user} {dc.domain} yes",
+            check=False,
+        )
+        self._executor.run("systemctl restart nginx", check=False)
+
+    def is_template_installed(self) -> bool:
+        """Check if both template files exist in the HestiaCP directory."""
+        return (
+            self._executor.file_exists(self.tpl_dest)
+            and self._executor.file_exists(self.stpl_dest)
+        )