feat: add orphan blob cleanup to Settings > Storage tab (#18)

- database.rs: add has_active_capture_jobs(), list_orphaned_blob_rows(),
  all_referenced_file_relpaths(), delete_orphaned_blob_rows()
- routes.rs: GET/DELETE /api/archives/:id/blob-cleanup (ROLE_ADMIN)
  - GET returns {orphaned_blob_rows, deletable_files, total_bytes}
  - DELETE has two active-capture guards (before and after disk walk)
    to prevent deleting files mid-capture; walks raw/ and raw_tweets/
  - Referenced set = entry_artifacts.relpath ∪ live blobs' raw_relpath,
    so a file is never deleted if any artifact still points at its path
- api.js: scanOrphanBlobs(), deleteOrphanBlobs()
- App.jsx: pass archiveId to SettingsView; add 'storage' to SETTINGS_TABS
  so /settings/storage survives refresh/back navigation
- SettingsView.jsx: new Storage tab (admin-only) with idle→scanning→
  scanned→deleting→done/error state machine; shows file/record counts
  and human-readable byte sizes before a btn-danger confirm

Tests (14 new, 248 total passing):
- database.rs: has_active_capture_jobs for pending/running/completed,
  list_orphaned_blob_rows, all_referenced_file_relpaths edge cases,
  delete_orphaned_blob_rows preserves referenced rows
- routes.rs: auth (401), active-capture 409 on GET and DELETE,
  end-to-end delete preserving referenced file and removing orphan
  blob file + extra disk-only file

frontend/src/App.jsx

@@ -18,7 +18,7 @@ import ToastStack from './components/ToastStack'
 export const AuthContext = createContext(null);
 
 const VIEWS = ['archive','tags','collections','runs','admin','settings']
-const SETTINGS_TABS = ['profile','tokens','instance']
+const SETTINGS_TABS = ['profile','tokens','instance','storage']
 
 function parseLocation() {
   const parts = window.location.pathname.split('/').filter(Boolean)
@@ -326,7 +326,7 @@ export default function App() {
               <CollectionsView archiveId={archiveId} />
             )}
             {view === 'settings' && (
-              <SettingsView tab={settingsTab} onTabChange={setSettingsTab} />
+              <SettingsView tab={settingsTab} onTabChange={setSettingsTab} archiveId={archiveId} />
             )}
           </div>
           <ContextRail