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/schemas/service_info.py

@@ -0,0 +1,29 @@
+from datetime import datetime
+
+from pydantic import BaseModel
+
+
+class ServiceInfoOut(BaseModel):
+    app_name: str
+    environment: str
+    api_prefix: str
+    app_version: str
+    hostname: str
+    python_version: str
+    platform: str
+    service_started_at: datetime
+    uptime_seconds: int
+    update_source: str
+    latest_version: str | None
+    latest_ref: str | None
+    update_available: bool
+    last_checked_at: datetime | None
+    last_check_error: str | None
+
+
+class ServiceInfoCheckResult(BaseModel):
+    latest_version: str | None
+    latest_ref: str | None
+    update_available: bool
+    last_checked_at: datetime
+    last_check_error: str | None