docs: define time-unit invalid bound error contract

documentation/entities/time_unit.md

@@ -15,6 +15,28 @@ The components that are used to change the upper or lower time bounds are `TimeB
 
 After the time unit is created, its ID (`tu_name`) is set to be the same as its name, but lowercase (for example, a time unit with the name "Abdounian" has the id "abdounian"). Because of this, changing an existing time unit's name causes a foreign key error in the database because the "now_time_unit" and "now_tau" tables are connected by it. When editing a time unit, the name field is currently disabled to not allow this error to happen.
 
+
+## Validation behavior for missing bounds
+
+For `PUT /time-unit`, `up_bnd` and `low_bnd` are validated against existing Time Bound records before write operations. If one or both referenced bounds are missing, the request is rejected with `403` and a structured validation array instead of returning `500`.
+
+Example response:
+
+```json
+[
+  {
+    "name": "Upper Bound",
+    "error": "Upper bound with ID 40000 does not exist"
+  },
+  {
+    "name": "Lower Bound",
+    "error": "Lower bound with ID 30000 does not exist"
+  }
+]
+```
+
+This contract is intentionally consumable by frontend validation/error-notification logic.
+
 ## Problems:
 
 At the moment it seems that Time Units get info about their updates from wrong places. Old PHP-version displays differring data on updates tab. It seems (I might be wrong about this) that PHP-version bases the info on the table now_time_update and the new one on the table now_tau. Prisma doesn't seem to handle the query if based on now_time_update with current schema, but the change should be doable with Prisma's rawSQL.