Make chat_type a native enum and test migrations with pytest-alembic

[lesnik512]

Supersedes #3, which GitHub auto-closed when its stacked base branch was deleted
on merge of #2. Same branch, rebased onto main.

Rationale in
planning/changes/2026-08-21.03-native-enum-and-migration-tests.md.

Native enum

sa.Enum(native_enum=False, create_constraint=True) produced a permanent
autogenerate false positive — Postgres reflects the CHECK body as
chat_type::text = ANY (ARRAY[...]), which never matches what Alembic renders
from the model, so every run proposed dropping ck_chats_chattype. alembic check could therefore never be a drift gate.

chat_type is now a native Postgres enum and migrations/env.py imports
alembic-postgresql-enum for its autogenerate hooks. The library wrote the
entire conversion migration, USING clause and working downgrade included.

Migration tests

tests/migrations/ runs four pytest-alembic built-ins:
test_single_head_revision, test_upgrade, test_up_down_consistency and
test_model_definitions_match_ddl (the last is alembic check as a test).
just test only ever ran downgrade base && upgrade head, which proves neither
per-revision reversibility nor the absence of branched heads.

The suite cycles the schema out from under db_session's rollback fixture, so
it is excluded from the default run (--ignore in addopts, omit in
coverage) and gets just test-migrations plus its own CI step.

Also

• Settings.sync_db_dsn_parsed owns the asyncpg → psycopg2 DSN rewrite that
  env.py and the alembic_engine fixture both need.
• alembic.ini gains path_separator = os; without it Alembic's
  DeprecationWarning fails tests under filterwarnings = ["error"].
• Second commit: pylint.max-args = 10 in the ruff config replaces eight
  # noqa: PLR0913, PLR0917 annotations. Litestar binds path/query/DI params by
  name and pytest binds fixtures by name, so handlers and tests legitimately
  exceed the default of five. max-positional-args defaults to max-args, so
  one setting covers both rules.

Verification

• just test — 109 passed, 100% coverage.
• just test-migrations — 4 passed.
• just lint, just check-planning — clean.
• alembic upgrade head && alembic check — clean (previously reported
  remove_constraint ck_chats_chattype); downgrade -1 && upgrade head
  round-trips.