feat: add unified webhook registry and multi-provider dispatcher

[rishii2208]

Summary

This PR introduces a unified inbound webhook system that routes platform webhooks to provider-specific handlers via a registry pattern.

Related Issue

Closes #1265

Problem

Postiz currently lacks a centralized, provider-agnostic mechanism for receiving real-time inbound events (likes, shares, comments).
Without this, each provider would require bespoke controller wiring, leading to duplication and poor scalability.

Solution

• Added a WebhookProvider interface defining the contract for inbound webhooks
• Introduced a webhook registry in IntegrationManager for O(1) provider lookup
• Implemented a single InboundWebhooksController that routes requests by provider name
• Added an example Facebook webhook provider to demonstrate usage

Why this matters

• Enables real-time engagement tracking without polling
• Keeps provider-specific logic isolated
• Makes adding new webhook providers trivial
• Aligns with Postiz’s multi-provider architecture

Happy to adjust based on feedback.