docs(data_store): document polymorphic for_id resolution pattern

Clarifies why v_data_store has no for_id_random JOIN column (for_type
is runtime-variable), how get_id_random() resolves the integer FK at
model validation time, and that Step 4 guarantees integers never leak
into API responses.

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

app/models/data_store_models.py

@@ -106,6 +106,10 @@ class Data_Store_Base(BaseModel):
                         values[f'{field}_random'] = resolved_rid
 
         # 3. Handle Polymorphic for_id
+        # v_data_store intentionally has no for_id_random JOIN column because for_type is
+        # runtime-variable (event, event_session, person, journal, etc.) — the target table
+        # is unknown at view-definition time.  Resolution falls back to get_id_random() here.
+        # If resolution fails, Step 4 strips the raw integer to None — integers never leak.
         if f_rid := values.get('for_id_random'):
             values['for_id'] = f_rid
         elif values.get('for_id') and values.get('for_type'):