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SQLite does not have a storage class set aside for storing dates
and/or times. Instead, the built-in Date And Time Functions of SQLite
are capable of storing dates and times as TEXT, REAL, or INTEGER
values
https://www.sqlite.org/datatype3.html#section_2_2
Thus this behavior is very different from PostgreSQL and mysql both of which happen to have separate data types for date, time and datetimes.
So the answer to “Is this behavior consistent across all Django database backends?” is no
“Where is this documented?” refer above link
“Also, how do I determine if a datetime in string form that I have received from an external source (not a Django form) will be valid with Django’s database?” It depends. If you have trouble with specific dates and times received from external sources, I would suggest a separate question on it.