feat(backend): bootstrap Go module, config, and data plane

Introduce configuration validation, Postgres migrations, Redis/storage adapters, and the seed sample video.

backend/internal/db/migrations/0005_avatar.sql

@@ -0,0 +1,3 @@
+ALTER TABLE users ADD COLUMN IF NOT EXISTS avatar_variant TEXT NOT NULL DEFAULT 'beam';
+ALTER TABLE users ADD COLUMN IF NOT EXISTS avatar_palette TEXT NOT NULL DEFAULT '';
+ALTER TABLE users ADD COLUMN IF NOT EXISTS avatar_photo TEXT NOT NULL DEFAULT '';