v0.4: licensing — server validation, DPAPI cache, Ed25519 signed responses

UX bonus: clicking "Reprendre" on an interrupted download skips the
release-notes confirmation dialog (the user already approved when they
first clicked Installer).

Server side
-----------
- migrations/001_init.sql: licenses, license_machines, rate_limit,
  audit_log on InnoDB/utf8mb4. Foreign keys, unique on license_key, slot
  uniqueness per (license_id, machine_id).
- api/lib/Db.php: thin PDO singleton (exception mode, prepared, no emul).
- api/lib/Crypto.php: Ed25519 sign/verify via libsodium (sodium_crypto_*),
  HMAC-SHA-256 helper for v0.6, canonicalJson() that strips `signature`
  before serializing — must match exactly the encoding done client-side
  before verify.
- api/routes/ValidateLicense.php: POST /license/validate. Looks up the
  key, walks the machine slot logic (insert or update last_seen),
  enforces max_machines, returns a payload signed Ed25519 + status of
  valid/expired/revoked/machine_limit_exceeded/invalid. Audit logs every
  outcome. Rate-limit 10/min/IP via the rate_limit table.
- tools/generate-keypair.php: prints a fresh sodium keypair so the
  operator drops the hex into config.php and the public_key_hex into the
  launcher resource.
- tools/issue-license.php: PRSRV-XXXX-XXXX-XXXX-XXXX generator (32-char
  unambiguous alphabet), inserts the license, prints the key once.
- tools/sign-manifest.php: now also signs the manifest itself with
  Ed25519 after computing the per-zip sha256s.
- config.example.php: schema rewritten with sections db / hmac / ed25519
  / jwt / rate-limit. config.php remains gitignored.

Client side
-----------
- Models/License.cs: LicenseValidationRequest + LicenseValidationResponse
  with CanDownload(VersionManifest) — entitlement_until vs version's
  minLicenseDate. The status valid|expired|revoked|machine_limit_exceeded
  flow is preserved end-to-end.
- Core/Licensing/LicenseService.cs:
  * machineId = SHA-256 of HKLM/Software/Microsoft/Cryptography/MachineGuid
    + UserName (stable, no PII leak)
  * online ValidateAsync calls /license/validate with launcher version
  * embedded server-pubkey.txt drives Ed25519 verification of the
    response (skipped gracefully if pubkey not yet provisioned)
  * SaveCached / GetCached use DPAPI CurrentUser scope on the license
    key; the cleartext key never touches disk
  * GetCached has a 7-day offline grace window after the last successful
    validation, so going offline doesn't lock the user out
- Core/Resources/server-pubkey.txt: EmbeddedResource. Default content is
  a comment, which the service treats as "no pubkey configured" and
  bypasses verification. Operator pastes the real hex post-deploy and
  rebuilds.
- Core/PSLauncher.Core.csproj: Polly, NSec.Cryptography (Ed25519),
  System.Security.Cryptography.ProtectedData (DPAPI).
- App/Views/OnboardingDialog.xaml(.cs): first-launch / "🔑 Activer"
  modal. Calls LicenseService, displays status messages with red
  foreground on errors and green-tinted secondary text otherwise.
- ViewModels/VersionRowViewModel.cs: new LicenseAllowsDownload property.
  Install button label switches to "🔒 License insuffisante" when the
  user's entitlement_until precedes the version's minLicenseDate;
  CanInstall is false in that case so the click is a no-op too.
- ViewModels/MainViewModel.cs: loads the cached license at startup (no
  network call), surfaces it as LicenseSummary in the top bar, exposes
  ActivateLicenseCommand to (re)open the onboarding dialog. RebuildList
  applies the per-version license filter so older installed versions
  remain launchable but newer-than-license ones can't be downloaded.
- Views/MainWindow.xaml: top bar gains a "🔑 Activer / changer" button
  next to the license summary.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/PSLauncher.App/Views/OnboardingDialog.xaml

@@ -0,0 +1,66 @@
+<Window x:Class="PSLauncher.App.Views.OnboardingDialog"
+        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+        Title="Activation PROSERVE Launcher"
+        Width="540" Height="420"
+        MinWidth="480" MinHeight="380"
+        WindowStartupLocation="CenterOwner"
+        ResizeMode="CanResize"
+        Background="{StaticResource Brush.Bg.Window}">
+    <Grid Margin="32,28">
+        <Grid.RowDefinitions>
+            <RowDefinition Height="Auto" />
+            <RowDefinition Height="Auto" />
+            <RowDefinition Height="Auto" />
+            <RowDefinition Height="Auto" />
+            <RowDefinition Height="*" />
+            <RowDefinition Height="Auto" />
+        </Grid.RowDefinitions>
+
+        <TextBlock Grid.Row="0" Text="Activer votre license"
+                   FontSize="22" FontWeight="SemiBold"
+                   Foreground="{StaticResource Brush.Text.Primary}" />
+
+        <TextBlock Grid.Row="1"
+                   Text="Saisis la clé fournie par ASTERION. Elle te donne accès aux téléchargements jusqu'à la date de validité associée."
+                   TextWrapping="Wrap"
+                   Foreground="{StaticResource Brush.Text.Secondary}"
+                   Margin="0,8,0,18" />
+
+        <TextBlock Grid.Row="2" Text="Clé de license"
+                   FontSize="12" FontWeight="Bold"
+                   Foreground="{StaticResource Brush.Text.Secondary}" />
+        <TextBox  Grid.Row="3" x:Name="KeyBox"
+                  Margin="0,6,0,16"
+                  Padding="10,8"
+                  FontFamily="Consolas" FontSize="14"
+                  CharacterCasing="Upper"
+                  Background="#1A1A20"
+                  Foreground="{StaticResource Brush.Text.Primary}"
+                  BorderBrush="{StaticResource Brush.Border}"
+                  BorderThickness="1"
+                  Text="PRSRV-XXXX-XXXX-XXXX-XXXX" />
+
+        <ScrollViewer Grid.Row="4" VerticalScrollBarVisibility="Auto">
+            <StackPanel x:Name="StatusPanel">
+                <TextBlock x:Name="StatusText"
+                           TextWrapping="Wrap"
+                           Foreground="{StaticResource Brush.Text.Secondary}" />
+            </StackPanel>
+        </ScrollViewer>
+
+        <StackPanel Grid.Row="5" Orientation="Horizontal" HorizontalAlignment="Right" Margin="0,18,0,0">
+            <Button Style="{StaticResource SecondaryButton}"
+                    Content="Plus tard"
+                    IsCancel="True"
+                    Margin="0,0,12,0"
+                    Click="OnLater" />
+            <Button Style="{StaticResource AccentButton}"
+                    Content="Activer"
+                    Padding="32,10"
+                    IsDefault="True"
+                    Click="OnActivate"
+                    x:Name="ActivateButton" />
+        </StackPanel>
+    </Grid>
+</Window>