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>

backend/app/config.py

@@ -127,6 +127,17 @@ class Settings(BaseSettings):
     # ERROR, CRITICAL). Bumped from INFO when chasing a problem.
     log_level: str = Field(default="INFO", env="LOG_LEVEL")
 
+    # Auth — JWT signing key. Set SECRET_KEY in .env for production.
+    # If unset, a deterministic fallback is used (acceptable for
+    # single-machine homelab deploys, but set a real key if the instance
+    # is network-exposed).
+    secret_key: str = Field(
+        default="mulita-dev-secret-change-me",
+        env="SECRET_KEY",
+    )
+    access_token_expire_minutes: int = Field(default=60, env="ACCESS_TOKEN_EXPIRE_MINUTES")
+    refresh_token_expire_days: int = Field(default=30, env="REFRESH_TOKEN_EXPIRE_DAYS")
+
     @property
     def cors_origins(self) -> list[str]:
         """Parse the ALLOWED_ORIGINS env var into a list. Accepts: