feat(backend): invite system + user management

- InviteToken model with unique token for each invite
- POST /users/invite - create invite by email with role (admin/mecanic)
- POST /auth/accept-invite - accept invite, set password, return JWT
- GET /users - list all users in tenant
- DELETE /users/{id} - deactivate user (cannot deactivate owner)
- Alembic migration for invites table
- 25 passing tests (auth + sync + orders + pdf + portal + invoices + users)

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

backend/alembic/versions/1a4da27efc65_add_invites_table.py

@@ -0,0 +1,44 @@
+"""add_invites_table
+
+Revision ID: 1a4da27efc65
+Revises: eec3c13599e7
+Create Date: 2026-03-13 17:36:58.364672
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = '1a4da27efc65'
+down_revision: Union[str, None] = 'eec3c13599e7'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.create_table('invites',
+    sa.Column('email', sa.String(length=200), nullable=False),
+    sa.Column('rol', sa.String(length=20), nullable=False),
+    sa.Column('token', sa.String(length=36), nullable=False),
+    sa.Column('used', sa.Text(), nullable=True),
+    sa.Column('id', sa.String(length=36), nullable=False),
+    sa.Column('tenant_id', sa.String(length=36), nullable=False),
+    sa.Column('created_at', sa.Text(), nullable=False),
+    sa.Column('updated_at', sa.Text(), nullable=False),
+    sa.PrimaryKeyConstraint('id')
+    )
+    op.create_index(op.f('ix_invites_tenant_id'), 'invites', ['tenant_id'], unique=False)
+    op.create_index(op.f('ix_invites_token'), 'invites', ['token'], unique=True)
+    # ### end Alembic commands ###
+
+
+def downgrade() -> None:
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.drop_index(op.f('ix_invites_token'), table_name='invites')
+    op.drop_index(op.f('ix_invites_tenant_id'), table_name='invites')
+    op.drop_table('invites')
+    # ### end Alembic commands ###