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

[lesnik512]

Stacked on #2 — base is refactor-di-wiring-and-chat-listing so the diff stays
clean; GitHub retargets this to main when #2 merges.

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

What

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.

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

Incidental

• 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"].

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.