Warn once when writing to temporary internal database

The internal database defaults to a temporary file that is deleted on
shutdown. Plugin writes that expect persistence currently vanish silently.

Detect such writes via an `internal_ephemeral_writes()` context manager
in `datasette.database`: Datasette wraps `_refresh_schemas` (its own
catalog/metadata writes) with the marker, and `Database.execute_write_fn`
prints a one-time stderr warning when a write hits a `is_temp_disk`
database without the marker set. The warning recommends restarting with
`--internal path/to/file.db` for persistence.

docs/internals.rst

@@ -2037,6 +2037,8 @@ Plugin authors are asked to practice good etiquette when using the internal data
 3. Use temporary tables or shared in-memory attached databases when possible.
 4. Avoid implementing features that could expose private data stored in the internal database by other plugins.
 
+If a plugin writes to the default temporary internal database, Datasette prints a one-time warning to standard error noting that the data will be lost on shutdown and recommending the ``--internal path/to/file.db`` flag for persistence. Datasette suppresses this warning for its own catalog and metadata writes, which are recreated on every startup.
+
 .. _internals_internal_schema:
 
 Internal database schema