fix(nested-crud): re-inject parent FK after model serialization to prevent 1364 errors

Root cause: child model root_validators (Vision ID anti-leakage guard) strip
integer IDs before they can be serialized into the INSERT dict, causing MariaDB
to reject the INSERT with 'Field does not have a default value' (1364).

Fix: re-inject resolved_parent_id into data_to_insert after validated_obj.dict()
in post_child_obj(). This is safe — the integer was already verified against the
DB before model validation.

Affected (were all broken since ~2026-01-27):
  - journal/{id}/journal_entry/
  - event/{id}/event_session/
  - event/{id}/event_person/
  - event/{id}/event_registration/
  - event/{id}/event_presenter/
  - event/{id}/event_presentation/
  - event/{id}/event_location/
  - event/{id}/event_track/
  - event/{id}/event_device/
  - event/{id}/event_abstract/
  - event/{id}/event_badge/ (different symptom: NULL FK)

Tests: add nested create lifecycle regression tests to test_e2e_v3_demo_parity.py
  - POST + Vision check + DELETE for journal/journal_entry and event/event_session
  - All 9 checks passing (7s)

Docs: update tests/README.md with accurate demo_parity description and
  a 'When to Run Tests' matrix to prevent future gaps in coverage.

app/routers/api_crud_v3_nested.py

@@ -46,7 +46,7 @@ async def get_child_obj_li(
         ):
     """
     List Child Objects (One-to-Many).
-    
+
     Retrieves a list of child objects associated with a specific parent.
     1. Verifies parent existence and user access to the parent.
     2. Filters children where `{parent_obj_type}_id` matches the parent's ID.
@@ -62,7 +62,7 @@ async def get_child_obj_li(
     and_like_dict_obj = None
     or_like_dict_obj = None
     and_in_dict_li_obj = None
-    
+
     jp_obj = safe_json_loads(urllib.parse.unquote(jp)) if jp else None
     if jp_obj:
         if jp_obj.get('qry'): qry_dict_li = jp_obj['qry']
@@ -74,7 +74,7 @@ async def get_child_obj_li(
 
     order_by_li = safe_json_loads(order_by_li)
     obj_name = child_obj_type
-    
+
     if obj_name not in obj_type_kv_li or parent_obj_type not in obj_type_kv_li:
         return mk_resp(data=False, status_code=400, response=response, status_message=f"Invalid object type(s).")
 
@@ -125,10 +125,10 @@ async def get_child_obj_li(
     if sql_result is False:
         # Standardized rich error bubbling
         db_err = format_db_error(get_last_sql_error())
-        
+
         # If it's a schema error (like Unknown Column), it's a 400 Bad Request
         status_code = 400 if db_err.category == "database_schema" else 500
-        
+
         return mk_resp(data=False, status_code=status_code, response=response, status_message="Listing failed due to database error.", details=db_err.dict())
 
     if sql_result:
@@ -155,7 +155,7 @@ async def search_child_obj_li(
         ):
     """
     Search Child Objects (POST).
-    
+
     Advanced search endpoint for nested objects.
     """
     from app.db_sql import redis_lookup_id_random, sql_select
@@ -239,7 +239,7 @@ async def post_child_obj(
         ):
     """
     Create Child Object.
-    
+
     1. Verifies Parent existence and access.
     2. Automatically links the new child to the parent (`{parent_obj_type}_id` = parent_id).
     3. Performs standard creation logic (validation, injection, sanitization).
@@ -295,6 +295,10 @@ async def post_child_obj(
 
     data_to_insert = validated_obj.dict(exclude_unset=True)
 
+    # Re-inject parent FK after model serialization. Some model root_validators strip
+    # integer IDs (a Vision ID anti-leakage guard) which would drop the FK from the dict.
+    data_to_insert[f'{parent_obj_type}_id'] = resolved_parent_id
+
     if sql_insert_result := sql_insert(data=data_to_insert, table_name=table_name_insert):
         new_obj_id = sql_insert_result
         new_obj_id_random = get_id_random(record_id=new_obj_id, table_name=child_obj_type)
@@ -324,7 +328,7 @@ async def get_child_obj(
         ):
     """
     Retrieve Child Object.
-    
+
     Verifies that the child belongs to the specified parent.
     """
     from app.db_sql import redis_lookup_id_random, sql_select
@@ -373,7 +377,7 @@ async def patch_child_obj(
         ):
     """
     Update Child Object.
-    
+
     Verifies that the child belongs to the specified parent before updating.
     """
     from app.db_sql import redis_lookup_id_random, sql_select, sql_update
@@ -435,7 +439,7 @@ async def get_child_obj(
         ):
     """
     Retrieve Child Object.
-    
+
     Verifies that the child belongs to the specified parent.
     """
     from app.db_sql import redis_lookup_id_random, sql_select
@@ -484,7 +488,7 @@ async def patch_child_obj(
         ):
     """
     Update Child Object.
-    
+
     Verifies that the child belongs to the specified parent before updating.
     """
     from app.db_sql import redis_lookup_id_random, sql_select, sql_update
@@ -545,7 +549,7 @@ async def delete_child_obj(
         ):
     """
     Delete Child Object.
-    
+
     Verifies that the child belongs to the specified parent before deleting.
     """
     from app.db_sql import redis_lookup_id_random, sql_select, sql_update, sql_delete