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/0016_invite_paused.sql

@@ -0,0 +1,8 @@
+-- Allow temporarily pausing registration invites without revoking them
+
+ALTER TABLE registration_invites
+    DROP CONSTRAINT IF EXISTS registration_invites_status_check;
+
+ALTER TABLE registration_invites
+    ADD CONSTRAINT registration_invites_status_check
+    CHECK (status IN ('active', 'paused', 'revoked', 'exhausted'));