feat(media): storage for images and audio in notes (M1)

- Table media (migration 5), files in $DATA_DIR/media/<docId>/.
- POST /documents/{id}/media (write access), POST .../media/fetch copies an
  external https image (SSRF-guarded, not in the demo), GET /media/{id}
  with range requests for readers of a note that shows the file.
- Format detected from the content (no SVG); own sandbox CSP on answers.
- Blocks point to files through their content (media:<id>), so the note
  sync needs no change; compaction removes files no block mentions any
  more after a day. Demo reset and demo start clear the folder.
- Limits MEDIA_MAX_IMAGE_MB, MEDIA_MAX_AUDIO_MB, MEDIA_QUOTA_MB,
  MEDIA_DEMO_MAX_MB.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

cmd/server/main.go

@@ -9,6 +9,7 @@ import (
 	"net/http"
 	"os"
 	"os/signal"
+	"path/filepath"
 	"syscall"
 	"time"
 	_ "time/tzdata" // time zones without system tzdata (scratch/alpine images)
@@ -46,6 +47,7 @@ func main() {
 
 	if cfg.DemoMode {
 		slog.Info("🧪 Demo-Modus aktiv: Setze Datenbank zurück und richte Demo-Accounts ein...")
+		_ = os.RemoveAll(filepath.Join(filepath.Dir(cfg.DatabasePath), "media")) // uploads of the last run
 		if err := store.ResetAndSeedDemo(); err != nil {
 			slog.Warn("⚠️ Fehler beim Einrichten des Demo-Modus", "error", err)
 		} else {