feat: Migrate Arbiter and Modpack Version Checker to monorepo

WHAT WAS DONE:
- Migrated Arbiter (discord-oauth-arbiter) code to services/arbiter/
- Migrated Modpack Version Checker code to services/modpack-version-checker/
- Created .env.example for Arbiter with all required environment variables
- Moved systemd service file to services/arbiter/deploy/
- Organized directory structure per Gemini monorepo recommendations

WHY:
- Consolidate all service code in one repository
- Prepare for Gemini code review (Panel v1.12 compatibility check)
- Enable service-prefixed Git tagging (arbiter-v2.1.0, modpack-v1.0.0)
- Support npm workspaces for shared dependencies

SERVICES MIGRATED:
1. Arbiter (Discord OAuth bot) - Originally written by Gemini + Claude
   - Full source code from ops-manual docs/implementation/
   - Created comprehensive .env.example
   - Ready for Panel v1.12 compatibility verification

2. Modpack Version Checker (Python CLI tool)
   - Full source code from ops-manual docs/tasks/
   - Written for Panel v1.11, needs Gemini review for v1.12
   - Never had code review before

STILL TODO:
- Whitelist Manager - Pull from Billing VPS (38.68.14.188)
  - Currently deployed and running
  - Needs Panel v1.12 API compatibility fix (Task #86)
  - Requires SSH access to pull code

NEXT STEPS:
- Gemini code review for Panel v1.12 API compatibility
- Create package.json for each service
- Test npm workspaces integration
- Deploy after verification

FILES:
- services/arbiter/ (25 new files, full application)
- services/modpack-version-checker/ (21 new files, full application)

Signed-off-by: The Golden Chronicler <claude@firefrostgaming.com>

services/modpack-version-checker/src/modpack_checker/notifier.py

@@ -0,0 +1,122 @@
+"""Notification delivery — Discord webhooks and generic HTTP webhooks."""
+
+from __future__ import annotations
+
+from typing import Optional
+
+import requests
+
+
+class NotificationError(Exception):
+    """Raised when a notification cannot be delivered."""
+
+
+class DiscordNotifier:
+    """Send modpack update alerts to a Discord channel via webhook.
+
+    Embeds are used so the messages look polished without additional
+    configuration on the user's side.
+    """
+
+    _EMBED_COLOR_UPDATE = 0xF5A623   # amber — update available
+    _EMBED_COLOR_TEST = 0x43B581     # green — test / OK
+
+    def __init__(self, webhook_url: str, timeout: int = 10) -> None:
+        self.webhook_url = webhook_url
+        self.timeout = timeout
+
+    # ------------------------------------------------------------------
+    # Public helpers
+    # ------------------------------------------------------------------
+
+    def send_update(
+        self,
+        modpack_name: str,
+        curseforge_id: int,
+        old_version: Optional[str],
+        new_version: str,
+    ) -> None:
+        """Post an update-available embed to Discord.
+
+        Raises:
+            NotificationError: if the webhook request fails.
+        """
+        embed = {
+            "title": "Modpack Update Available",
+            "color": self._EMBED_COLOR_UPDATE,
+            "url": f"https://www.curseforge.com/minecraft/modpacks/{curseforge_id}",
+            "fields": [
+                {
+                    "name": "Modpack",
+                    "value": modpack_name,
+                    "inline": True,
+                },
+                {
+                    "name": "CurseForge ID",
+                    "value": str(curseforge_id),
+                    "inline": True,
+                },
+                {
+                    "name": "Previous Version",
+                    "value": old_version or "Unknown",
+                    "inline": False,
+                },
+                {
+                    "name": "New Version",
+                    "value": new_version,
+                    "inline": False,
+                },
+            ],
+            "footer": {
+                "text": "Modpack Version Checker \u2022 Firefrost Gaming",
+            },
+        }
+        self._post({"embeds": [embed]})
+
+    def test(self) -> None:
+        """Send a simple test message to verify the webhook URL works.
+
+        Raises:
+            NotificationError: if the request fails.
+        """
+        self._post(
+            {
+                "embeds": [
+                    {
+                        "title": "Webhook Connected",
+                        "description": (
+                            "Modpack Version Checker notifications are working correctly."
+                        ),
+                        "color": self._EMBED_COLOR_TEST,
+                        "footer": {"text": "Modpack Version Checker \u2022 Firefrost Gaming"},
+                    }
+                ]
+            }
+        )
+
+    # ------------------------------------------------------------------
+    # Internal
+    # ------------------------------------------------------------------
+
+    def _post(self, payload: dict) -> None:
+        try:
+            response = requests.post(
+                self.webhook_url,
+                json=payload,
+                timeout=self.timeout,
+            )
+            # Discord returns 204 No Content on success
+            if response.status_code not in (200, 204):
+                raise NotificationError(
+                    f"Discord returned HTTP {response.status_code}: {response.text[:200]}"
+                )
+        except requests.ConnectionError as exc:
+            raise NotificationError(
+                "Could not reach Discord. Check your internet connection."
+            ) from exc
+        except requests.Timeout:
+            raise NotificationError(
+                f"Discord webhook timed out after {self.timeout}s."
+            )
+        except requests.RequestException as exc:
+            raise NotificationError(f"Webhook request failed: {exc}") from exc