New media webhook events and v1 routing

We have added media-lifecycle webhooks to the webhooks service so you can react to upload, encoding, and readiness events without polling.

Highlights:

• Media webhook handling and validation — A new dispatch path validates the media event payload and routes it through the same retry, signing, and delivery machinery as other webhook events.
• API for storing media logs — Each delivery is recorded with timestamp, response code, and error context so you can audit the lifecycle of any media asset.
• /v1 root path — All webhook routes have moved under /v1 for consistency with the rest of the FastPix API. The pre-v1 path is still accepted during a deprecation window.
• Robust special-character handling — Webhook payloads containing special characters (apostrophes, emoji, multi-byte glyphs) are now serialized with explicit charset=utf-8 so receivers do not need to second-guess encoding.

If you already consume FastPix webhooks, no action is needed; the new media events appear in the same event list and use your existing secret.