Add service information feature with version checks

This commit introduces a new "Service Information" section displaying runtime details, installed version, and update status for the NexaPG application. It includes backend API endpoints, database schema changes, and a corresponding frontend page that allows users to check for updates against the official repository. The `.env` example now includes an `APP_VERSION` variable, and related documentation has been updated.

backend/app/models/models.py

@@ -147,6 +147,25 @@ class EmailNotificationSettings(Base):
     )
 
 
+class ServiceInfoSettings(Base):
+    __tablename__ = "service_info_settings"
+
+    id: Mapped[int] = mapped_column(Integer, primary_key=True)
+    current_version: Mapped[str] = mapped_column(String(64), nullable=False, default="0.1.0")
+    release_check_url: Mapped[str | None] = mapped_column(String(500), nullable=True)
+    latest_version: Mapped[str | None] = mapped_column(String(64), nullable=True)
+    update_available: Mapped[bool] = mapped_column(Boolean, nullable=False, default=False)
+    last_checked_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True), nullable=True)
+    last_check_error: Mapped[str | None] = mapped_column(Text, nullable=True)
+    created_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), nullable=False, server_default=func.now())
+    updated_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True),
+        nullable=False,
+        server_default=func.now(),
+        onupdate=func.now(),
+    )
+
+
 class AlertNotificationEvent(Base):
     __tablename__ = "alert_notification_events"
     __table_args__ = (UniqueConstraint("alert_key", "target_id", "severity", name="uq_alert_notif_event_key_target_sev"),)