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/migrate.go

@@ -0,0 +1,61 @@
+package db
+
+import (
+	"context"
+	"fmt"
+	"io/fs"
+	"sort"
+	"strings"
+
+	"github.com/jackc/pgx/v5"
+	"github.com/jackc/pgx/v5/pgxpool"
+)
+
+// Migrate runs all *.sql files in the migrations fs in lexicographic order.
+// Tracks applied migrations in schema_migrations.
+func Migrate(ctx context.Context, pool *pgxpool.Pool, migrations fs.FS) error {
+	if _, err := pool.Exec(ctx, `CREATE TABLE IF NOT EXISTS schema_migrations (version TEXT PRIMARY KEY, applied_at TIMESTAMPTZ NOT NULL DEFAULT now())`); err != nil {
+		return fmt.Errorf("create migrations table: %w", err)
+	}
+	entries, err := fs.ReadDir(migrations, ".")
+	if err != nil {
+		return err
+	}
+	var names []string
+	for _, e := range entries {
+		if !e.IsDir() && strings.HasSuffix(e.Name(), ".sql") {
+			names = append(names, e.Name())
+		}
+	}
+	sort.Strings(names)
+
+	for _, name := range names {
+		applied, err := pool.Exec(ctx, `SELECT 1 FROM schema_migrations WHERE version=$1`, name)
+		if err != nil {
+			return err
+		}
+		if applied.RowsAffected() > 0 {
+			continue
+		}
+		body, err := fs.ReadFile(migrations, name)
+		if err != nil {
+			return err
+		}
+		tx, err := pool.BeginTx(ctx, pgx.TxOptions{})
+		if err != nil {
+			return err
+		}
+		if _, err := tx.Exec(ctx, string(body)); err != nil {
+			_ = tx.Rollback(ctx)
+			return fmt.Errorf("apply %s: %w", name, err)
+		}
+		if _, err := tx.Exec(ctx, `INSERT INTO schema_migrations (version) VALUES ($1)`, name); err != nil {
+			_ = tx.Rollback(ctx)
+			return err
+		}
+		if err := tx.Commit(ctx); err != nil {
+			return err
+		}
+	}
+	return nil
+}