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/db.go

@@ -0,0 +1,19 @@
+package db
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/jackc/pgx/v5/pgxpool"
+)
+
+func Connect(ctx context.Context, url string) (*pgxpool.Pool, error) {
+	pool, err := pgxpool.New(ctx, url)
+	if err != nil {
+		return nil, fmt.Errorf("connect db: %w", err)
+	}
+	if err := pool.Ping(ctx); err != nil {
+		return nil, fmt.Errorf("ping db: %w", err)
+	}
+	return pool, nil
+}