feat: multi-user auth with per-user media isolation

Introduce username/password authentication with admin and user roles.
Each user gets their own media directory under /photos/{username}/ with
isolated photos, folders, heaps, and tags. Admins manage users and
observe the full library from a dedicated Settings page.

Backend:
- User model with bcrypt passwords and JWT access/refresh tokens
- Auth router (login, refresh, setup, change-password, status)
- Admin router (user CRUD with last-admin protection)
- user_id FK added to photos, folders, source_roots, heaps, tags
- All data routers scoped by authenticated user
- Scanner inherits user_id from source root owner
- Thumbnails stored under user-prefixed paths for isolation
- Library endpoints accept ?scope=global for admin cross-user view
- Alembic migration 0009 with data migration for existing installs
- Defensive bootstrap.py handles fresh vs existing DB startup

Frontend:
- AuthContext with token lifecycle, auto-refresh, login/logout
- Login page, first-run setup page, auth gate in App.tsx
- Bearer token interceptor on all API requests
- User identity + logout in left sidebar
- Admin-only Settings page with Library Management and Users tabs
- UserManagement panel (add, edit role, reset password, deactivate)
- Settings shows global stats across all users for admin
- Filter bar, right sidebar, keyboard hints hidden on settings page

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/layout/LeftSidebar.tsx

@@ -20,6 +20,9 @@ import {
   Users,
   Eye,
   EyeOff,
+  User as UserIcon,
+  LogOut,
+  Shield,
 } from 'lucide-react'
 import clsx from 'clsx'
 import { sourceFolders, photos as photosApi, type FolderTreeNode } from '../../services/api'
@@ -40,6 +43,7 @@ import {
 import { registerUndoable } from '../../store/undoStore'
 import type { Photo } from '../../types/photo'
 import { DeleteFolderDialog } from '../dialogs/DeleteFolderDialog'
+import { useAuth } from '../../contexts/AuthContext'
 
 interface TreeItem {
   id: string
@@ -55,10 +59,10 @@ interface TreeItem {
 
 interface LeftSidebarProps {
   onCollapse: () => void
-  onOpenSettings: () => void
 }
 
-export function LeftSidebar({ onCollapse, onOpenSettings }: LeftSidebarProps) {
+export function LeftSidebar({ onCollapse }: LeftSidebarProps) {
+  const { user, isAdmin, logout } = useAuth()
   const [expandedItems, setExpandedItems] = useState<Set<string>>(new Set(['library', 'folders', 'heaps']))
   // Inline rename state for source-root rows. Stores the id being edited
   // and the draft name. Double-click a folder row to start.
@@ -797,17 +801,40 @@ export function LeftSidebar({ onCollapse, onOpenSettings }: LeftSidebarProps) {
         <HeapsPanel />
       </div>
 
-      {/* Settings entry point — pinned to the bottom of the panel so it
-       *  sits out of the way of the library tree but is always reachable. */}
-      <div className="border-t border-border p-1.5">
-        <button
-          onClick={onOpenSettings}
-          className="flex w-full items-center gap-2 rounded px-2 py-1 text-[12px] text-text-muted hover:bg-surface-2 hover:text-text"
-          title="Settings"
-        >
-          <Settings className="h-3.5 w-3.5" />
-          Settings
-        </button>
+      {/* Bottom panel — user identity + settings, pinned below the tree. */}
+      <div className="border-t border-border p-1.5 space-y-0.5">
+        {/* User row */}
+        <div className="flex items-center gap-2 rounded px-2 py-1 text-[12px] text-text-muted">
+          <UserIcon className="h-3.5 w-3.5 flex-shrink-0" />
+          <span className="flex-1 truncate text-text">{user?.username}</span>
+          {isAdmin && (
+            <span className="rounded bg-accent/20 px-1 py-px text-[10px] leading-none text-accent flex-shrink-0">
+              <Shield className="inline h-2.5 w-2.5" />
+            </span>
+          )}
+          <button
+            onClick={logout}
+            className="rounded p-0.5 text-text-muted hover:bg-surface-2 hover:text-red-400 flex-shrink-0"
+            title="Sign out"
+          >
+            <LogOut className="h-3 w-3" />
+          </button>
+        </div>
+
+        {/* Settings — admin only, navigates to the settings section */}
+        {isAdmin && (
+          <button
+            onClick={() => navigateToSection('settings', {})}
+            className={clsx(
+              'flex w-full items-center gap-2 rounded px-2 py-1 text-[12px] hover:bg-surface-2 hover:text-text',
+              currentSection === 'settings' ? 'text-primary' : 'text-text-muted',
+            )}
+            title="Settings"
+          >
+            <Settings className="h-3.5 w-3.5" />
+            Settings
+          </button>
+        )}
       </div>
 
       <DeleteFolderDialog