[Postiz] Unified webhook registry for inbound platform events

### 🔖 Feature description Currently there's no unified way to receive real-time webhooks from external platforms (Facebook, X, LinkedIn, etc.). Each provider would need bespoke endpoint wiring, duplicating controller logic, verification, and error handling across many files. This proposes a registry-based inbound webhook system: - A WebhookProvider interface that any platform can implement - A provider registry inside IntegrationManager (Map-based, O(1) lookup) - A single InboundWebhooksController at /inbound-webhooks/:provider that dispatches to the correct handler - Optional verifyWebhook() for platforms that sign payloads (e.g., Facebook's HMAC-SHA256) Adding a new provider becomes 3 lines of wiring , no new controllers, no duplicated logic. ### 🎤 Why is this feature needed ? Polling external platforms for engagement data (likes, shares, comments) is inefficient and burns through API rate limits. Most social platforms offer webhook subscriptions for real-time event delivery,