#237 fix: give auto temporal columns a database default in migrations

[RAprogramm]

Closes #237

A column marked #[auto] is excluded from the generated INSERT, but the DDL emitted DEFAULT only when one was written by hand. A table created from MIGRATION_UP therefore rejected the first create() with 23502 not-null violation — including for the entity shape used in the README quick start.

Non-nullable #[auto] columns with a temporal type now receive the matching clock default:

SQL type	Default
TIMESTAMPTZ, TIMESTAMP	NOW()
DATE	CURRENT_DATE
TIME, TIMETZ	CURRENT_TIME

An explicit #[column(default = "...")] still wins. Nullable auto columns, arrays and non-temporal types are untouched — the macro has no meaningful value to invent there, and a nullable column already accepts the absent value.

Verification

The live-Postgres entity from #234 drops its #[column(default = "NOW()")] workaround, so crud_roundtrip now fails without this fix and passes with it. Three unit tests cover the clock defaults, the explicit-default precedence, and the untouched cases.

Docs

#[auto] documentation updated in the README attribute reference and in the attribute page of all five wiki languages.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ All tests successful. No failed tests found.

📢 Thoughts on this report? Let us know!