fix(models): add cascade delete-orphan to parent-child relationships

Without cascade, SQLAlchemy tried to NULL-out foreign keys on child rows
before deleting the parent, hitting NOT NULL constraints in PostgreSQL.

- Routine.steps: cascade="all, delete-orphan" (routine_steps.routine_id)
- MedicationEntry.usage_history: cascade="all, delete-orphan"
  (medication_usages.medication_record_id)

Product.inventory already had cascade set correctly.
No DB migration needed — ORM-level only.

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

backend/innercontext/models/health.py

@@ -36,7 +36,10 @@ class MedicationEntry(SQLModel, table=True):
         ),
     )
 
-    usage_history: list["MedicationUsage"] = Relationship(back_populates="medication")
+    usage_history: list["MedicationUsage"] = Relationship(
+        back_populates="medication",
+        sa_relationship_kwargs={"cascade": "all, delete-orphan"},
+    )
 
 
 class MedicationUsage(SQLModel, table=True):

backend/innercontext/models/routine.py

@@ -38,7 +38,10 @@ class Routine(SQLModel, table=True):
         ),
     )
 
-    steps: List["RoutineStep"] = Relationship(back_populates="routine")
+    steps: List["RoutineStep"] = Relationship(
+        back_populates="routine",
+        sa_relationship_kwargs={"cascade": "all, delete-orphan"},
+    )
 
 
 class GroomingSchedule(SQLModel, table=True):