feat: show disk usage of mover source with configurable warning threshold

GET /api/storage/disk-usage returns source path stats (total/used/free bytes, %).
Polls every 30s while Storage page is open.

Backend:
- migration 0009: adds mover_warning_threshold (1-99, default 80) to storage_config
- StatPath moved to internal/storage package for testability
- handleStorageDiskUsage returns {source: PathStat}

Frontend:
- DiskUsageBar component in Mergerfs Mover card: usage bar with color
  green < threshold, amber >= threshold, red >= 95%
- Warning message when threshold reached or exceeded
- New input in config form: Umbral de aviso (%)
- storageDiskUsage() API method

Tests: StatPath unit tests (empty/nonexistent/valid paths)

Version: 0.7.3

web/src/api.ts

@@ -169,6 +169,7 @@ export interface StorageConfig {
   mover_remove_source: boolean;
   mover_inplace: boolean;
   mover_rsync_options: string;
+  mover_warning_threshold: number;
   snapraid_content: string;
   snapraid_data_dirs: string;
   snapraid_parity_dir: string;
@@ -176,6 +177,20 @@ export interface StorageConfig {
   updated_at: string;
 }
 
+export interface PathStat {
+  path: string;
+  available: boolean;
+  total_bytes: number;
+  used_bytes: number;
+  free_bytes: number;
+  used_percent: number;
+  error?: string;
+}
+
+export interface StorageDiskUsage {
+  source: PathStat;
+}
+
 export interface StorageJob {
   id: number;
   kind: JobKind;
@@ -300,6 +315,7 @@ export const api = {
   storageCapabilities: () => request<StorageCapabilities>("GET", "/storage/capabilities"),
   getStorageConfig: () => request<StorageConfig>("GET", "/storage/config"),
   updateStorageConfig: (c: Partial<StorageConfig>) => request<StorageConfig>("PUT", "/storage/config", c),
+  storageDiskUsage: () => request<StorageDiskUsage>("GET", "/storage/disk-usage"),
   listStorageJobs: (limit = 50) => request<{ jobs: StorageJob[] }>("GET", `/storage/jobs?limit=${limit}`),
   getStorageJob: (id: number) => request<StorageJob>("GET", `/storage/jobs/${id}`),
   startStorageJob: (kind: JobKind, args?: Record<string, unknown>) =>