feat(backend): FastAPI + libSQL + auth register/login/me + tests (TDD)

- FastAPI app with lifespan, CORS, health endpoint
- SQLAlchemy 2.0 async with aiosqlite, Base/UUIDMixin/TenantMixin/TimestampMixin
- Tenant and User models with multi-tenant isolation
- Auth: register (creates tenant+user), login, /me endpoint
- JWT HS256 tokens, bcrypt password hashing
- Alembic async setup with initial migration
- 6 passing tests (register, login, wrong password, me, no token, health)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/alembic/env.py

@@ -0,0 +1,50 @@
+import asyncio
+from logging.config import fileConfig
+
+from alembic import context
+from sqlalchemy.ext.asyncio import create_async_engine
+
+from app.config import settings
+from app.db.base import Base
+import app.db.models  # noqa: F401
+
+config = context.config
+if config.config_file_name is not None:
+    fileConfig(config.config_file_name)
+
+target_metadata = Base.metadata
+
+
+def run_migrations_offline() -> None:
+    url = settings.DATABASE_URL
+    context.configure(
+        url=url,
+        target_metadata=target_metadata,
+        literal_binds=True,
+        dialect_opts={"paramstyle": "named"},
+    )
+    with context.begin_transaction():
+        context.run_migrations()
+
+
+def do_run_migrations(connection):
+    context.configure(connection=connection, target_metadata=target_metadata)
+    with context.begin_transaction():
+        context.run_migrations()
+
+
+async def run_async_migrations() -> None:
+    connectable = create_async_engine(settings.DATABASE_URL)
+    async with connectable.connect() as connection:
+        await connection.run_sync(do_run_migrations)
+    await connectable.dispose()
+
+
+def run_migrations_online() -> None:
+    asyncio.run(run_async_migrations())
+
+
+if context.is_offline_mode():
+    run_migrations_offline()
+else:
+    run_migrations_online()