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/0013_registration.sql

@@ -0,0 +1,13 @@
+-- Gated public registration: IP one-shot + admin toggle
+
+CREATE TABLE IF NOT EXISTS registration_ips (
+    ip         TEXT PRIMARY KEY,
+    user_id    BIGINT NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    created_at TIMESTAMPTZ NOT NULL DEFAULT now()
+);
+
+CREATE INDEX IF NOT EXISTS registration_ips_user_idx ON registration_ips (user_id);
+
+INSERT INTO app_settings (key, value) VALUES
+    ('registration_enabled', 'false')
+ON CONFLICT (key) DO NOTHING;