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/auth.py

@@ -0,0 +1,47 @@
+"""
+Authentication utilities — password hashing and JWT token management.
+"""
+from datetime import datetime, timedelta, timezone
+
+from jose import jwt, JWTError
+from passlib.context import CryptContext
+
+from app.config import settings
+
+pwd_context = CryptContext(schemes=["bcrypt"], deprecated="auto")
+
+ALGORITHM = "HS256"
+
+
+def hash_password(plain: str) -> str:
+    return pwd_context.hash(plain)
+
+
+def verify_password(plain: str, hashed: str) -> bool:
+    return pwd_context.verify(plain, hashed)
+
+
+def create_access_token(user_id: str, role: str) -> str:
+    expire = datetime.now(timezone.utc) + timedelta(minutes=settings.access_token_expire_minutes)
+    payload = {
+        "sub": user_id,
+        "role": role,
+        "exp": expire,
+        "type": "access",
+    }
+    return jwt.encode(payload, settings.secret_key, algorithm=ALGORITHM)
+
+
+def create_refresh_token(user_id: str) -> str:
+    expire = datetime.now(timezone.utc) + timedelta(days=settings.refresh_token_expire_days)
+    payload = {
+        "sub": user_id,
+        "exp": expire,
+        "type": "refresh",
+    }
+    return jwt.encode(payload, settings.secret_key, algorithm=ALGORITHM)
+
+
+def decode_token(token: str) -> dict:
+    """Decode and validate a JWT. Raises JWTError on any problem."""
+    return jwt.decode(token, settings.secret_key, algorithms=[ALGORITHM])