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.

README.md

@@ -14,6 +14,7 @@ It combines FastAPI, React, and PostgreSQL in a Docker Compose stack with RBAC,
 - [Make Commands](#make-commands)
 - [Configuration Reference (`.env`)](#configuration-reference-env)
 - [Core Functional Areas](#core-functional-areas)
+- [Service Information](#service-information)
 - [Target Owner Notifications](#target-owner-notifications)
 - [API Overview](#api-overview)
 - [`pg_stat_statements` Requirement](#pg_stat_statements-requirement)
@@ -141,6 +142,7 @@ Note: Migrations run automatically when the backend container starts (`entrypoin
 | Variable | Description |
 |---|---|
 | `APP_NAME` | Application display name |
+| `APP_VERSION` | Displayed NexaPG version in Service Information |
 | `ENVIRONMENT` | Runtime environment (`dev`, `staging`, `prod`, `test`) |
 | `LOG_LEVEL` | Backend log level (`DEBUG`, `INFO`, `WARNING`, `ERROR`) |
 
@@ -230,6 +232,13 @@ Recommended values for `VITE_API_URL`:
   - from email + from name
   - recipient test mail
 
+### Service Information
+
+- Sidebar entry for runtime and system details
+- Displays current version, latest known version, uptime, host, and platform
+- "Check for Updates" against the official upstream repository (`git.nesterovic.cc/nessi/NexaPG`)
+- Version/update source are read-only in UI (maintainer-controlled in code/release flow)
+
 ## Target Owner Notifications
 
 Email alert routing is target-specific:
@@ -288,6 +297,11 @@ Email alert routing is target-specific:
 - `PUT /api/v1/admin/settings/email`
 - `POST /api/v1/admin/settings/email/test`
 
+### Service Information
+
+- `GET /api/v1/service/info`
+- `POST /api/v1/service/info/check`
+
 ## `pg_stat_statements` Requirement
 
 Query Insights requires `pg_stat_statements` on the monitored target: