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/capabilities.go

@@ -0,0 +1,34 @@
+package storage
+
+import (
+	"context"
+
+	"github.com/darroyo/nasctl/internal/db"
+	"github.com/darroyo/nasctl/internal/system"
+)
+
+type Capabilities struct {
+	Rsync           bool `json:"rsync"`
+	MergerfsBin     bool `json:"mergerfs"`
+	MergerfsMounted bool `json:"mergerfs_mounted"`
+	SnapraidBin     bool `json:"snapraid"`
+}
+
+func Probe(ctx context.Context) (Capabilities, error) {
+	rsyncOut, _, _ := system.Run(ctx, "command", "-v", "rsync")
+	snapraidOut, _, _ := system.Run(ctx, "command", "-v", "snapraid")
+	mergerfsOut, _, _ := system.Run(ctx, "command", "-v", "mergerfs")
+
+	_, _, mergerfsMountedErr := system.Run(ctx, "grep", "-q", "fuse.mergerfs", "/proc/mounts")
+
+	return Capabilities{
+		Rsync:           len(rsyncOut) > 0,
+		MergerfsBin:     len(mergerfsOut) > 0,
+		MergerfsMounted: mergerfsMountedErr == nil,
+		SnapraidBin:     len(snapraidOut) > 0,
+	}, nil
+}
+
+func ValidatePaths(cfg db.StorageConfig) error {
+	return nil
+}