Types: CLOB, NCHAR, NVARCHAR, DATETIME and DATE compile to DDL CrateDB cannot parse

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Problem

Five core SQLAlchemy types compile to type names CrateDB rejects, so CREATE TABLE fails with an opaque server error. Measured against CrateDB 6.4.3 with SQLAlchemy 2.0.50 on main:

SQLAlchemy type	Emitted DDL	Server response
sa.CLOB	CLOB	SQLParseException[Cannot find data type: clob]
sa.NCHAR(5)	NCHAR(5)	SQLParseException[Cannot find data type: nchar]
sa.NVARCHAR(50)	NVARCHAR(50)	SQLParseException[Cannot find data type: nvarchar]
sa.DATETIME	DATETIME	SQLParseException[Cannot find data type: datetime]
sa.DATE	DATE	UnsupportedFeatureException[Type 'date' does not support storage]

In each case the generic lower-case counterpart already works. sa.Text, sa.String, sa.DateTime and sa.Date all compile fine. Only the upper-case / SQL-standard spellings fall through to GenericTypeCompiler and emit the ANSI name verbatim.

This affects anyone using the SQL-standard type names directly, and anything generating models from a generic schema.

One design question: DATE

CrateDB gained a storable DATE column type in 6.5; on 6.4 and earlier the server rejects it with Type 'date' does not support storage.

Mapping sa.DATE → TIMESTAMP works on every supported server version and keeps sa.Date and sa.DATE on the same storage type but it means we never adopt the native type. The alternative is a version-gated mapping.

Discovered while working on #12 and moved changes on separate PR.
CI can't currently cannot test these, see the CI issue #303.