Remove configurable APP_VERSION and define it in code

The APP_VERSION variable is no longer configurable via the `.env` file or environment settings. Instead, the version is now hardcoded in `backend/app/core/config.py` as `NEXAPG_VERSION` and accessed through a property. This change simplifies version control and ensures consistency across deployments.

README.md

@@ -142,7 +142,6 @@ 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`) |
 
@@ -238,6 +237,7 @@ Recommended values for `VITE_API_URL`:
 - Displays current version, latest known version, uptime, host, and platform
 - "Check for Updates" against the latest published release in the official upstream repository (`git.nesterovic.cc/nessi/NexaPG`)
 - Version/update source are read-only in UI (maintainer-controlled in code/release flow)
+- Local displayed version is code-defined in `backend/app/core/config.py` (`NEXAPG_VERSION`) and not configurable via `.env`
 
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