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>

.env.example

@@ -36,6 +36,20 @@ BACKEND_PORT=8001
 REDIS_PORT=6379
 
 
+# ── AUTH ─────────────────────────────────────────────────────────────────────
+
+# Secret key used to sign JWT tokens. Generate a strong random value for
+# production (e.g. `openssl rand -base64 32`). The default is a deterministic
+# placeholder acceptable only for local/homelab use.
+# SECRET_KEY=change-me-to-a-random-string
+
+# How long access and refresh tokens stay valid. Access tokens are short-lived
+# and silently refreshed by the frontend; refresh tokens let a session survive
+# across browser restarts.
+# ACCESS_TOKEN_EXPIRE_MINUTES=60
+# REFRESH_TOKEN_EXPIRE_DAYS=30
+
+
 # ── CORS ─────────────────────────────────────────────────────────────────────
 
 # Comma-separated list of allowed origins for direct browser access to the