Enforce single result per modelinput

In Acidwatch we allow for a simulation consisting of multiple model
inputs (chaining models), and for each modelinput there corresponds a
modelresult. This was added as part of the orm, but we should enforce it
also on the db level.

Author: lars-petter-hauge

backend/alembic/versions/a3f8b2c91d40_add_unique_constraint_on_results_model_.py

@@ -0,0 +1,31 @@
+"""add unique constraint on results.model_input_id
+
+Revision ID: a3f8b2c91d40
+Revises: 7c2e1f4b8a90
+Create Date: 2026-06-16 00:00:00.000000
+
+Enforces the intended one-to-one relationship between model_inputs and
+results at the database level. The ORM already declares this as a singular
+``Mapped[ModelResult | None]`` relationship, but previously no unique
+constraint prevented duplicate results for a single model_input.
+"""
+
+from typing import Sequence, Union
+
+from alembic import op
+
+
+revision: str = "a3f8b2c91d40"
+down_revision: Union[str, Sequence[str], None] = "7c2e1f4b8a90"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.create_unique_constraint(
+        "uq_results_model_input_id", "results", ["model_input_id"]
+    )
+
+
+def downgrade() -> None:
+    op.drop_constraint("uq_results_model_input_id", "results", type_="unique")

backend/src/acidwatch_api/database.py

@@ -67,7 +67,9 @@ class ModelInput(Base):
 class ModelResult(Base):
     __tablename__ = "results"
 
-    model_input_id: Mapped[UUID] = mapped_column(ForeignKey("model_inputs.id"))
+    model_input_id: Mapped[UUID] = mapped_column(
+        ForeignKey("model_inputs.id"), unique=True
+    )
     concentrations: Mapped[dict[str, float]] = mapped_column(JSON)
     panels: Mapped[list[Any]] = mapped_column(JSON)
     error: Mapped[str | None] = mapped_column()