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/migrations/0008_storage.sql

@@ -0,0 +1,31 @@
+CREATE TABLE IF NOT EXISTS storage_config (
+    id INTEGER PRIMARY KEY CHECK (id = 1),
+    mover_source          TEXT NOT NULL DEFAULT '',
+    mover_dest            TEXT NOT NULL DEFAULT '',
+    mover_clean_macos     INTEGER NOT NULL DEFAULT 1,
+    mover_remove_source   INTEGER NOT NULL DEFAULT 1,
+    mover_inplace         INTEGER NOT NULL DEFAULT 1,
+    mover_rsync_options   TEXT NOT NULL DEFAULT '',
+    snapraid_content      TEXT NOT NULL DEFAULT '',
+    snapraid_data_dirs    TEXT NOT NULL DEFAULT '',
+    snapraid_parity_dir   TEXT NOT NULL DEFAULT '',
+    snapraid_scrub_plan   INTEGER NOT NULL DEFAULT 8,
+    updated_at            TEXT NOT NULL DEFAULT (datetime('now'))
+);
+
+CREATE TABLE IF NOT EXISTS storage_jobs (
+    id          INTEGER PRIMARY KEY AUTOINCREMENT,
+    kind        TEXT NOT NULL,
+    status      TEXT NOT NULL DEFAULT 'queued',
+    args_json   TEXT NOT NULL DEFAULT '{}',
+    pid         INTEGER NOT NULL DEFAULT 0,
+    started_at  TEXT,
+    finished_at TEXT,
+    exit_code   INTEGER NOT NULL DEFAULT -1,
+    output      TEXT NOT NULL DEFAULT '',
+    error       TEXT NOT NULL DEFAULT '',
+    created_at  TEXT NOT NULL DEFAULT (datetime('now'))
+);
+
+CREATE INDEX IF NOT EXISTS idx_storage_jobs_created ON storage_jobs(created_at DESC);
+CREATE INDEX IF NOT EXISTS idx_storage_jobs_kind_created ON storage_jobs(kind, created_at DESC);