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/app/db/base.py

@@ -0,0 +1,32 @@
+import uuid
+from datetime import datetime, UTC
+
+from sqlalchemy import String, Text
+from sqlalchemy.orm import DeclarativeBase, Mapped, mapped_column
+
+
+def uuid7() -> str:
+    return str(uuid.uuid4())
+
+
+class Base(DeclarativeBase):
+    pass
+
+
+class UUIDMixin:
+    id: Mapped[str] = mapped_column(String(36), primary_key=True, default=uuid7)
+
+
+class TenantMixin:
+    tenant_id: Mapped[str] = mapped_column(String(36), nullable=False, index=True)
+
+
+class TimestampMixin:
+    created_at: Mapped[str] = mapped_column(
+        Text, default=lambda: datetime.now(UTC).isoformat()
+    )
+    updated_at: Mapped[str] = mapped_column(
+        Text,
+        default=lambda: datetime.now(UTC).isoformat(),
+        onupdate=lambda: datetime.now(UTC).isoformat(),
+    )