Add OpenAPI spec for KillrVideo v2 monolith backend

Captures the OpenAPI 3.1.0 spec from the running service (v2.0.0-alpha-local),
covering 33 endpoints across users, videos, search, comments, ratings,
recommendations, flags, and moderation.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: pmcfadin