feat(python-codegen): @column field naming, server-default exprs, --entities allowlist

[dmealing]

Motivation

Surfaced by generating Python record models from the same entity metadata that drives the TypeScript Drizzle layer (one cross-language source of truth). Three consumer-facing gaps blocked that, all in the Python port's entity_model generator + CLI — no metamodel/registry change, so no cross-port conformance reconciliation is needed.

Changes

1. @column-aware Pydantic field naming. The generator emitted field.name verbatim, so an entity authored camelCase for the TS port (callPurpose + @column: call_purpose) produced camelCase Python fields that don't match the DB columns. The Pydantic field name is now field.column ?? field.name — one entity feeds both languages idiomatically (camelCase TS property, snake_case Python field = the physical column). Backward-compatible: a model with no @column emits byte-identically.

2. Server-side default expressions are no longer rendered as Python literal defaults. A string @default matching the SQL-expression patterns (now, now(), current_timestamp/date/time, anything ending in () such as gen_random_uuid()) is DB-filled — so id: uuid.UUID = "gen_random_uuid()" was invalid. Such a field now carries no Python default (keeps its required/optional shape). Literal defaults (bool/int/str/enum) are unchanged. Mirrors the TS column-mapper.ts SQL_EXPR_PATTERNS so both ports agree on what counts as an expression.

3. metaobjects gen --entities <names> allowlist. The whole model is still loaded (so extends bases and @objectRef VOs resolve), but only the named entities are emitted — generate a subset of a shared model without splitting the metadata. Wired through gen and verify --codegen; run_gen already supported entity_filter.

Tests

New regression tests in test_entity_model.py (column override, fallback, server-default-expr vs literal) and test_cli.py (entities allowlist emit + verify in sync). Full tests/codegen suite green except two pre-existing test_cli_staleness_nudge failures unrelated to this change (they fail on clean origin/main too).

Also bumps the Python port to 0.12.0 (backward-compatible minor; aligns with the 0.12.x line).