docs(orm): document subquery selects (select_sub, add_select, table)

[tmgbedu]

Documents the query builder additions from fastapi-startkit/fastapi-startkit-framework#170 (released in v0.60.0).

Adds a Subquery selects section to docs/database/models.md under Querying:

• last_login_at example with its generated SQL
• select_sub(subquery, alias): builder or callable input, binding order, TypeError for anything else
• add_select(*columns): variadic, single list, dedup, {alias: subquery} dict (seeds {table}.* when nothing is selected), string values stay plain columns, bare subquery raises TypeError
• Migration note: add_select(alias, callable) was removed, use select_sub(callable, alias)
• table(name)

SQL output was checked with to_sql() (Postgres grammar) and against test_select_sub_grammar.py / test_sqlite_select_sub.py. npm run docs:build passes.

🤖 Generated with Claude Code