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/models/tenant.py

@@ -0,0 +1,18 @@
+from sqlalchemy import String, Text
+from sqlalchemy.orm import Mapped, mapped_column
+
+from app.db.base import Base, UUIDMixin, TimestampMixin
+
+
+class Tenant(Base, UUIDMixin, TimestampMixin):
+    __tablename__ = "tenants"
+    nume: Mapped[str] = mapped_column(String(200))
+    cui: Mapped[str | None] = mapped_column(String(20))
+    reg_com: Mapped[str | None] = mapped_column(String(30))
+    adresa: Mapped[str | None] = mapped_column(Text)
+    telefon: Mapped[str | None] = mapped_column(String(20))
+    email: Mapped[str | None] = mapped_column(String(200))
+    iban: Mapped[str | None] = mapped_column(String(34))
+    banca: Mapped[str | None] = mapped_column(String(100))
+    plan: Mapped[str] = mapped_column(String(20), default="trial")
+    trial_expires_at: Mapped[str | None] = mapped_column(Text)