Webhook URL derived from gateway URL via fragile string replace

[magent-cryptograss]

Problem

In delivery-kid/pinning-service/app/routes/content.py (line ~306), the Coconut webhook URL is derived from the IPFS gateway URL by stripping ipfs. from the hostname:

base_url = settings.ipfs_gateway_url.replace("ipfs.", "", 1)

So https://ipfs.delivery-kid.cryptograss.live becomes https://delivery-kid.cryptograss.live.

If the gateway URL format ever changes (different subdomain scheme, path-based routing, different domain entirely), the webhook URL silently goes nowhere and Coconut jobs complete into the void with no notification.

Proposed fix

Add a webhook_base_url (or public_base_url) config field to Settings. The webhook URL should be explicitly configured, not reverse-engineered from the gateway URL.

At minimum, add an assertion that the derived URL looks sane before using it:

assert base_url.startswith("https://"), f"Webhook base URL looks wrong: {base_url}"
assert "ipfs" not in base_url.split("/")[2], f"Webhook URL still contains 'ipfs': {base_url}"

Context

From PR #69 review. The Coconut webhook is how we learn that transcoding completed — if it doesn't fire, HLS outputs sit on Coconut's servers and we never pin them.