feat(orm): add std::optional mapper value conversion

[GaspardKirira]

Summary

Support std::optional<T> values at the ORM mapper boundary.

Empty optionals should map to SQL NULL, while populated optionals should use normal value conversion.

Current behavior

any_to_dbvalue_or_throw() supports direct scalar values and explicit NULL values.

A mapper returning std::optional<std::string> or std::optional<std::int64_t> is rejected as an unsupported std::any type.

Expected behavior

Common optional scalar types must convert predictably without requiring every mapper to unwrap them manually.

Scope

• Support optional signed integers.
• Support optional unsigned integers.
• Support optional floating-point values.
• Support optional booleans.
• Support optional strings.
• Support optional BLOB values.
• Map empty optionals to NULL.
• Add conversion tests.
• Preserve existing direct values.

Suggested files

• modules/orm/include/vix/orm/db_compat.hpp
• modules/orm/tests/optional_conversion_test.cpp
• modules/orm/docs/...

Acceptance criteria

• Empty optionals convert to vix::db::null().
• Populated optionals preserve their value.
• Unsupported optional inner types fail clearly.
• Existing conversion behavior remains unchanged.
• No optional references are accepted.
• Tests cover every documented optional type.

Non-goals

This issue does not include:

• automatic row-to-optional conversion;
• nested optionals;
• nullable relationship loading;
• changing DbValue.

Suggested labels

• scope:orm
• type:feature
• tests
• good first issue
• status:accepted