feat: Discord webhook notifications for live + VOD events

For users who run the app on a dedicated archive box and aren't
watching the queue panel directly. Three optional event types post
to a Discord webhook:

- Live recording started: red embed with streamer + URL + output
  filename. Fires inside downloadLiveStream after chat-capture
  init, before streamlink launches, so a hung streamlink doesn't
  silently delay the alert.
- Live recording ended: green (ok) or purple (failed) embed with
  duration, file size, captured-chat-message count, output filename.
  Fires after streamlink exits — picks up cancellation, integrity
  failure, and clean stream-ended exits the same way.
- VOD download complete: green embed with file count + total bytes.
  Skipped for live items (those have their own end-of-recording
  embed; double-firing would be noisy).

Server:
- New isAcceptableDiscordWebhook(url) regex sanity-check —
  refuses URLs that aren't discord.com/api/webhooks/* so a
  pasted-by-mistake other URL doesn't leak data anywhere.
- sendDiscordWebhook(payload) is fire-and-forget: 8s timeout,
  errors logged via appendDebugLog but never surface to the user.
  Should NOT block the recording flow.
- DiscordEmbedColor enum maps live/success/info to known palette
  values (red / green / Twitch purple).
- Embed body slices fields to Discord's documented length limits
  (title 256, description 4096, field name 256, field value 1024,
  max 25 fields per embed) so a runaway long stream title can't
  produce a rejected webhook.

Renderer / settings:
- New Settings card "Discord-Webhook" between Backup and Updates.
  URL input + 3 toggles (live-start / live-end / vod-complete).
  All three default off, URL empty — totally inert until the user
  configures it.
- AppConfig type, autosave fingerprint, syncSettingsForm,
  applyLanguageToStaticUI, debounced-save IDs all updated. Webhook
  URL is debounced like other text inputs so each keystroke
  doesn't trigger a save.
- DE + EN locales for every label.

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

src/index.html

@@ -596,6 +596,29 @@
                         <pre id="debugLogOutput" class="log-panel">Lade...</pre>
                     </div>
 
+                    <div class="settings-card">
+                        <h3 id="discordCardTitle">Discord-Webhook</h3>
+                        <p id="discordCardIntro" style="color: var(--text-secondary); font-size:13px; margin-bottom:12px; line-height:1.5;">Sende Benachrichtigungen an einen Discord-Channel via Webhook — nuetzlich fuer Multi-Device-Setups oder eine dedizierte Archiv-Maschine.</p>
+                        <div class="form-group">
+                            <label id="discordWebhookUrlLabel">Webhook-URL</label>
+                            <input type="text" id="discordWebhookUrl" placeholder="https://discord.com/api/webhooks/...">
+                        </div>
+                        <div class="form-group">
+                            <label style="display:flex; align-items:center; gap:8px;">
+                                <input type="checkbox" id="discordNotifyLiveStartToggle">
+                                <span id="discordNotifyLiveStartLabel">Bei Live-Aufnahme-Start benachrichtigen</span>
+                            </label>
+                            <label style="display:flex; align-items:center; gap:8px; margin-top: 8px;">
+                                <input type="checkbox" id="discordNotifyLiveEndToggle">
+                                <span id="discordNotifyLiveEndLabel">Bei Live-Aufnahme-Ende benachrichtigen</span>
+                            </label>
+                            <label style="display:flex; align-items:center; gap:8px; margin-top: 8px;">
+                                <input type="checkbox" id="discordNotifyVodCompleteToggle">
+                                <span id="discordNotifyVodCompleteLabel">Bei abgeschlossenem VOD-Download benachrichtigen</span>
+                            </label>
+                        </div>
+                    </div>
+
                     <div class="settings-card">
                         <h3 id="backupCardTitle">Sicherung &amp; Wartung</h3>
                         <p id="backupCardIntro" style="color: var(--text-secondary); font-size:13px; margin-bottom:12px; line-height:1.5;">Konfiguration sichern, auf einem anderen Geraet wiederherstellen, oder die Liste der bereits heruntergeladenen VODs zuruecksetzen.</p>