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/0007_hearts_limit.sql

@@ -0,0 +1,10 @@
+-- Keep the database invariant aligned with the application-level hearts guard.
+UPDATE users
+SET hearts = 3,
+    is_locked = true,
+    locked_at = COALESCE(locked_at, now()),
+    hearts_updated_at = now()
+WHERE hearts > 3;
+
+ALTER TABLE users
+    ADD CONSTRAINT users_hearts_max CHECK (hearts BETWEEN 0 AND 3);