[ZogStriP]

Previously, the webhook checked its mutually exclusive event types with separate if blocks, and the playback lookup serialized a relation to read a single row by its unique id. The webhook specs pasted the same signed request, with a misspelled Content-Type, into every example and defined mux_signature on Object for the whole suite, and the :video fabricator used a fixed id, so specs had to pass one to fabricate a second video.

This change:

• turns the webhook into a case over the event type and reads the playback data with find_by
• uses staff? in can_upload_video?
• adds a post_webhook spec helper, scopes mux_signature to the webhook specs, uses enable_current_plugin and sequences the :video fabricator id
• renames the "allows only verified requests" spec after what it checks, that a signed event of a type the webhook doesn't handle is acknowledged
• drops the assertion on the error body of the blank secret rejection, since that body comes from core's Discourse::InvalidAccess handling, and keeps the checks on the 403 and the unchanged video

Part of a stack of PRs, to be merged in order:

1. FIX: Show a poster for Mux videos and play them inline on mobile #103
2. FIX: Update the right posts when a Mux video becomes ready #104
3. DEV: Simplify the webhook, playback lookup and their specs #105 👈 this PR
4. PERF: Cut per-post queries and table scans for post videos #106
5. FIX: Render each video in a post with its own status #107
6. DEV: Simplify video rendering and consolidate its tests #108
7. UX: Use sentence case for video strings and fix the preview placeholder #109
8. FIX: Show video changes in posts without reloading the topic #110
9. PERF: Load video streams only once playback starts #111
10. DEV: Move video rendering into a module #112
11. FIX: Restore video embeds in chat messages #113