feat: implement Phase 6 — Software Deployment

Backend:
- SoftwarePackage model (Name, Version, OsType, PackageManager, PackageName, InstallerUrl, Checksum, SilentArgs)
- RmmDbContext: SoftwarePackages DbSet + unique index on (Name, Version, OsType)
- SoftwarePackagesController: full CRUD with OsType filter
- DeployController: POST /api/v1/deploy creates InstallSoftware/UninstallSoftware TaskItem
- EF Migration: AddSoftwarePackages (20260319130448)

Go Agent:
- internal/deployer/deployer.go: Install() and Uninstall() with:
  - Chocolatey (Windows), apt/dnf (Linux), auto-detect
  - Direct installer fallback: HTTP download + SHA256 verify + silent install
  - Supports .msi, .exe (Windows) and .deb, .rpm (Linux)
- main.go: COMMAND_TYPE_INSTALL_SOFTWARE and COMMAND_TYPE_UNINSTALL_SOFTWARE routed to deployer

Frontend:
- SoftwarePage: Katalog tab (CRUD, OS filter, smart package manager select) + Deploy tab
- api/types.ts: SoftwarePackage, PackageManager, DeployRequest/Response types
- api/client.ts: softwarePackagesApi and deployApi
- App.tsx: Software nav item with Package icon

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Backend/src/NexusRMM.Core/Models/SoftwarePackage.cs

@@ -0,0 +1,26 @@
+namespace NexusRMM.Core.Models;
+
+public class SoftwarePackage
+{
+    public int Id { get; set; }
+    public string Name { get; set; } = string.Empty;
+    public string Version { get; set; } = string.Empty;
+    public OsType OsType { get; set; }
+
+    /// <summary>Paketmanager: "choco", "apt", "dnf", "direct"</summary>
+    public string PackageManager { get; set; } = "choco";
+
+    /// <summary>Paketname für den Paketmanager (z.B. "7zip" für choco)</summary>
+    public string PackageName { get; set; } = string.Empty;
+
+    /// <summary>Optionale direkte Download-URL (für Fallback)</summary>
+    public string? InstallerUrl { get; set; }
+
+    /// <summary>SHA256-Prüfsumme der Installer-Datei</summary>
+    public string? Checksum { get; set; }
+
+    /// <summary>Silent-Install-Parameter für direkten Installer</summary>
+    public string? SilentArgs { get; set; }
+
+    public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
+}