feat: add mergerfs mover and snapraid integration

New 'Almacenamiento' page with:
- Auto-detection of rsync, mergerfs, snapraid binaries and mergerfs mount
- Configurable pool settings (source/dest, macOS cleanup, rsync flags)
- Mergerfs mover with dry-run preview and live SSE output streaming
- SnapRAID diff/sync/scrub/check with live SSE output
- Async job system (1 concurrent job) with SSE streaming
- Job history table

Backend:
- internal/storage/ package with capabilities, mergerfs, snapraid, jobs
- storage_config and storage_jobs DB tables (migration 0008)
- GET/PUT /api/storage/config, GET /api/storage/capabilities
- POST/GET /api/storage/jobs, GET /api/storage/jobs/{id}/stream
- Storage operations disabled when NASCTL_EXEC_SYSTEM=false

Closes #new-feature

internal/storage/snapraid.go

@@ -0,0 +1,70 @@
+package storage
+
+import (
+	"fmt"
+	"os"
+	"strconv"
+	"strings"
+)
+
+type SnapraidConfig struct {
+	Content   string
+	DataDirs  []string
+	ParityDir string
+	ScrubPlan int
+}
+
+func BuildSnapraidArgs(kind string, cfg SnapraidConfig) ([]string, error) {
+	if cfg.Content == "" {
+		return nil, fmt.Errorf("snapraid content file is required")
+	}
+	args := []string{"snapraid", "-c", cfg.Content}
+	switch kind {
+	case "snapraid_diff":
+		args = append(args, "diff")
+	case "snapraid_sync":
+		args = append(args, "sync")
+	case "snapraid_scrub":
+		args = append(args, "scrub", "-p", strconv.Itoa(cfg.ScrubPlan))
+	case "snapraid_check":
+		args = append(args, "check")
+	default:
+		return nil, fmt.Errorf("unknown snapraid kind: %s", kind)
+	}
+	return args, nil
+}
+
+func ValidateSnapraidContent(content string) error {
+	if content == "" {
+		return fmt.Errorf("snapraid content file is required")
+	}
+	info, err := os.Stat(content)
+	if err != nil {
+		return fmt.Errorf("snapraid content %s: %w", content, err)
+	}
+	if info.IsDir() {
+		return fmt.Errorf("snapraid content %s: is a directory, not a file", content)
+	}
+	return nil
+}
+
+func ValidateScrubPlan(plan int) error {
+	if plan < 1 || plan > 99 {
+		return fmt.Errorf("scrub plan must be between 1 and 99, got %d", plan)
+	}
+	return nil
+}
+
+func ParseSnapraidDataDirs(raw string) []string {
+	if raw == "" {
+		return nil
+	}
+	var dirs []string
+	for _, d := range strings.Split(raw, ",") {
+		d = strings.TrimSpace(d)
+		if d != "" {
+			dirs = append(dirs, d)
+		}
+	}
+	return dirs
+}