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/models/folders.py

@@ -10,13 +10,16 @@ from app.database import Base
 
 class SourceRoot(Base):
     __tablename__ = 'source_roots'
-    
+
     id = Column(String, primary_key=True, default=lambda: str(uuid.uuid4()))
     name = Column(String, nullable=False)
     path = Column(String, unique=True, nullable=False)
     is_active = Column(Boolean, default=True)
     added_at = Column(DateTime, server_default=func.now())
-    
+
+    # Owner
+    user_id = Column(String, ForeignKey('users.id'), nullable=True, index=True)
+
     # Relationships
     folders = relationship("Folder", back_populates="source_root")
 
@@ -28,6 +31,9 @@ class Folder(Base):
     path = Column(String, unique=True, nullable=False)
     parent_id = Column(String, ForeignKey('folders.id'))
     source_root_id = Column(String, ForeignKey('source_roots.id'))
+
+    # Owner
+    user_id = Column(String, ForeignKey('users.id'), nullable=True, index=True)
     photo_count = Column(Integer, default=0)
     last_scanned = Column(DateTime)