Phase 1: make ORM/migrations Postgres-compatible

Dialect-neutral fixes that keep SQLite behaviour unchanged:
- event_observer: choose INSERT construct by bind dialect so the
  on_conflict upsert works on both SQLite and Postgres
- database: map postgresql:// -> postgresql+asyncpg:// for async sessions
  (was sqlite-only), via a _to_async_url helper
- models: import JSON from sqlalchemy (generic) not the sqlite dialect
- alembic/env: render_as_batch only for SQLite (its ALTER TABLE workaround)

Full SQLite test suite green (1045 passed) and fresh db upgrade builds all
tables; black/flake8/mypy clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

alembic/env.py

@@ -58,7 +58,9 @@ def run_migrations_offline() -> None:
         target_metadata=target_metadata,
         literal_binds=True,
         dialect_opts={"paramstyle": "named"},
-        render_as_batch=True,  # SQLite batch mode for ALTER TABLE
+        # Batch mode is a SQLite-only workaround for its limited ALTER TABLE;
+        # Postgres performs ALTERs directly.
+        render_as_batch=url.startswith("sqlite"),
     )
 
     with context.begin_transaction():