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/db/models.go

@@ -87,3 +87,39 @@ type ApplyLogEntry struct {
 	Success   bool      `json:"success"`
 	CreatedAt time.Time `json:"created_at"`
 }
+
+type StorageConfig struct {
+	ID                int64  `json:"id"`
+	MoverSource       string `json:"mover_source"`
+	MoverDest         string `json:"mover_dest"`
+	MoverCleanMacOS   bool   `json:"mover_clean_macos"`
+	MoverRemoveSource bool   `json:"mover_remove_source"`
+	MoverInplace      bool   `json:"mover_inplace"`
+	MoverRsyncOptions string `json:"mover_rsync_options"`
+	SnapraidContent   string `json:"snapraid_content"`
+	SnapraidDataDirs  string `json:"snapraid_data_dirs"`
+	SnapraidParityDir string `json:"snapraid_parity_dir"`
+	SnapraidScrubPlan int    `json:"snapraid_scrub_plan"`
+	UpdatedAt         string `json:"updated_at"`
+}
+
+type StorageJob struct {
+	ID         int64   `json:"id"`
+	Kind       string  `json:"kind"`
+	Status     string  `json:"status"`
+	ArgsJSON   string  `json:"args_json"`
+	PID        int     `json:"pid"`
+	StartedAt  *string `json:"started_at,omitempty"`
+	FinishedAt *string `json:"finished_at,omitempty"`
+	ExitCode   int     `json:"exit_code"`
+	Output     string  `json:"output"`
+	Error      string  `json:"error"`
+	CreatedAt  string  `json:"created_at"`
+}
+
+type StorageCapabilities struct {
+	Rsync           bool `json:"rsync"`
+	MergerfsBin     bool `json:"mergerfs"`
+	MergerfsMounted  bool `json:"mergerfs_mounted"`
+	SnapraidBin     bool `json:"snapraid"`
+}