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/models.py

@@ -0,0 +1,184 @@
+# -*- coding: utf-8 -*-
+"""
+Domain models and value objects for the HestiaCP Django-Gunicorn deployment engine.
+
+These dataclasses serve as the **Single Source of Truth** for all path
+conventions, service naming rules, and configuration parameters used across
+every module.  Centralising them here eliminates magic strings and satisfies
+the DRY principle.
+"""
+
+from __future__ import annotations
+
+import os
+from dataclasses import dataclass, field
+from enum import Enum, auto
+from pathlib import Path
+from typing import List, Optional
+
+
+# ---------------------------------------------------------------------------
+# Enumerations
+# ---------------------------------------------------------------------------
+
+class ProjectLayout(Enum):
+    """Detected Django project directory layout."""
+
+    MODULAR = auto()       # config/wsgi.py  (clean / modern)
+    TRADITIONAL = auto()   # <project>/wsgi.py  (django-admin startproject)
+    FLAT = auto()           # wsgi.py at project root (rare but valid)
+    UNKNOWN = auto()
+
+
+class DatabaseEngine(Enum):
+    """Database back-end detected in settings."""
+
+    SQLITE = auto()
+    POSTGRESQL = auto()
+    MYSQL = auto()
+    OTHER = auto()
+    UNKNOWN = auto()
+
+
+class DiagnosticStatus(Enum):
+    """Result status for a single diagnostic check."""
+
+    OK = auto()
+    WARNING = auto()
+    CRITICAL = auto()
+
+
+# ---------------------------------------------------------------------------
+# Path & naming conventions  (Single Source of Truth)
+# ---------------------------------------------------------------------------
+
+HESTIA_TEMPLATE_DIR = Path("/usr/local/hestia/data/templates/web/nginx")
+SYSTEMD_SERVICE_DIR = Path("/etc/systemd/system")
+TEMPLATE_NAME = "django-gunicorn"
+SOCKET_FILENAME = "gunicorn.sock"
+DEFAULT_VENV_CANDIDATES = ("venv", ".venv", "env")
+DEFAULT_STATIC_DIR = "staticfiles"
+
+
+def build_home_dir(user: str) -> Path:
+    """Return the HestiaCP home directory for *user*."""
+    return Path(f"/home/{user}")
+
+
+def build_web_dir(user: str, domain: str) -> Path:
+    """Return the web-root (``public_html``) for a domain."""
+    return build_home_dir(user) / "web" / domain / "public_html"
+
+
+def build_socket_path(user: str, domain: str) -> Path:
+    """Return the canonical unix-socket path."""
+    return build_web_dir(user, domain) / SOCKET_FILENAME
+
+
+def build_service_name(domain: str) -> str:
+    """Return the systemd service unit name (without ``.service``)."""
+    return f"gunicorn-{domain}"
+
+
+def build_service_file(domain: str) -> Path:
+    """Return the absolute path to the systemd unit file."""
+    return SYSTEMD_SERVICE_DIR / f"{build_service_name(domain)}.service"
+
+
+# ---------------------------------------------------------------------------
+# Core data-transfer objects
+# ---------------------------------------------------------------------------
+
+@dataclass(frozen=True)
+class DomainConfig:
+    """Immutable value object describing a HestiaCP domain deployment target."""
+
+    domain: str
+    user: str
+
+    @property
+    def home_dir(self) -> Path:
+        return build_home_dir(self.user)
+
+    @property
+    def web_dir(self) -> Path:
+        return build_web_dir(self.user, self.domain)
+
+    @property
+    def socket_path(self) -> Path:
+        return build_socket_path(self.user, self.domain)
+
+    @property
+    def service_name(self) -> str:
+        return build_service_name(self.domain)
+
+    @property
+    def service_file(self) -> Path:
+        return build_service_file(self.domain)
+
+
+@dataclass
+class DjangoProjectInfo:
+    """Result of inspecting the Django project inside ``public_html``."""
+
+    layout: ProjectLayout = ProjectLayout.UNKNOWN
+    wsgi_module: str = ""
+    venv_path: Optional[Path] = None
+    settings_module: str = ""
+    static_root_name: str = DEFAULT_STATIC_DIR
+    database_engine: DatabaseEngine = DatabaseEngine.UNKNOWN
+    has_dot_env: bool = False
+    has_requirements: bool = False
+    errors: List[str] = field(default_factory=list)
+
+    @property
+    def is_valid(self) -> bool:
+        return self.layout != ProjectLayout.UNKNOWN and bool(self.wsgi_module)
+
+
+@dataclass
+class ServiceConfig:
+    """All parameters needed to render a systemd unit file."""
+
+    domain_config: DomainConfig
+    project_info: DjangoProjectInfo
+    workers: int = 0
+    threads: int = 2
+    umask: str = "007"
+    log_level: str = "info"
+    restart_sec: int = 3
+
+    def __post_init__(self) -> None:
+        if self.workers <= 0:
+            cpu = os.cpu_count() or 1
+            self.workers = (2 * cpu) + 1
+
+
+@dataclass
+class DiagnosticCheck:
+    """A single diagnostic check result."""
+
+    name: str
+    status: DiagnosticStatus
+    message: str
+    suggestion: str = ""
+
+
+@dataclass
+class DiagnosticReport:
+    """Collection of diagnostic checks for a deployment."""
+
+    domain_config: DomainConfig
+    checks: List[DiagnosticCheck] = field(default_factory=list)
+
+    @property
+    def is_healthy(self) -> bool:
+        return all(c.status == DiagnosticStatus.OK for c in self.checks)
+
+    @property
+    def critical_count(self) -> int:
+        return sum(1 for c in self.checks if c.status == DiagnosticStatus.CRITICAL)
+
+    @property
+    def warning_count(self) -> int:
+        return sum(1 for c in self.checks if c.status == DiagnosticStatus.WARNING)