feat(backend): bootstrap Go module, config, and data plane

Introduce configuration validation, Postgres migrations, Redis/storage adapters, and the seed sample video.

backend/internal/storage/sample.go

@@ -0,0 +1,56 @@
+package storage
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+)
+
+const SampleVideoKey = "sample.mp4"
+
+// EnsureSampleMP4 uploads the color-bars sample video when the storage key is missing.
+// candidates are filesystem paths checked in order (first readable wins).
+func EnsureSampleMP4(stg Storage, candidates ...string) error {
+	if stg == nil {
+		return fmt.Errorf("storage is nil")
+	}
+	if stg.Exists(SampleVideoKey) {
+		return nil
+	}
+	for _, path := range candidates {
+		if path == "" {
+			continue
+		}
+		f, err := os.Open(path)
+		if err != nil {
+			continue
+		}
+		info, err := f.Stat()
+		if err != nil {
+			f.Close()
+			continue
+		}
+		if info.IsDir() || info.Size() == 0 {
+			f.Close()
+			continue
+		}
+		_, err = stg.Put(SampleVideoKey, f, info.Size())
+		f.Close()
+		if err != nil {
+			return fmt.Errorf("upload %s: %w", SampleVideoKey, err)
+		}
+		return nil
+	}
+	return fmt.Errorf("%s missing in storage and no readable candidate found among %v", SampleVideoKey, candidates)
+}
+
+// DefaultSampleCandidates returns common relative locations for the checked-in sample.
+func DefaultSampleCandidates() []string {
+	cwd, _ := os.Getwd()
+	return []string{
+		filepath.Join(cwd, "media", SampleVideoKey),
+		filepath.Join(cwd, "backend", "media", SampleVideoKey),
+		filepath.Join("media", SampleVideoKey),
+		filepath.Join("backend", "media", SampleVideoKey),
+	}
+}