feat(sprint2): инфраструктура БД — SQLAlchemy 2.0 async + Alembic

Первый кусок Спринта 2: подключаем SQLite через SQLAlchemy 2.0 (async,
ORM-стиль) и Alembic для миграций. Схема выбрана под будущий рост —
в threads сразу заведены nullable user_id и agent_config_id, чтобы
Спринты 3+ не тащили миграции задним числом.

- requirements.txt: sqlalchemy[asyncio]==2.0.36, aiosqlite==0.20.0,
  alembic==1.14.0.
- config: database_url + sqlite_path (./data/sqlite/app.db).
- db/base.py: DeclarativeBase; db/session.py: async engine,
  async_sessionmaker, get_session — FastAPI-dependency.
- db/models/Thread: id, name, user_id?, agent_config_id?, created_at,
  updated_at; relationship messages с cascade all, delete-orphan.
- db/models/Message: id, thread_id FK CASCADE, role, text, sources_json,
  assembled_prompt, created_at.
- Alembic инициализирован через async-шаблон, env.py доработан:
  sys.path, url из settings, target_metadata = Base.metadata.
- Начальная миграция e7199587be4b применена, таблицы threads/messages
  с индексами на FK и nullable-колонки созданы в data/sqlite/app.db.
- .gitignore: исключаем data/sqlite/ (БД — артефакт, не исходник).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

migrations/env.py

@@ -0,0 +1,90 @@
+import asyncio
+import os
+import sys
+from logging.config import fileConfig
+
+from sqlalchemy import pool
+from sqlalchemy.engine import Connection
+from sqlalchemy.ext.asyncio import async_engine_from_config
+
+from alembic import context
+
+sys.path.insert(0, os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
+
+from config import settings  # noqa: E402
+from db.base import Base  # noqa: E402
+from db import models  # noqa: F401, E402 — регистрация моделей в Base.metadata
+
+config = context.config
+config.set_main_option("sqlalchemy.url", settings.database_url)
+
+if config.config_file_name is not None:
+    fileConfig(config.config_file_name)
+
+target_metadata = Base.metadata
+
+# other values from the config, defined by the needs of env.py,
+# can be acquired:
+# my_important_option = config.get_main_option("my_important_option")
+# ... etc.
+
+
+def run_migrations_offline() -> None:
+    """Run migrations in 'offline' mode.
+
+    This configures the context with just a URL
+    and not an Engine, though an Engine is acceptable
+    here as well.  By skipping the Engine creation
+    we don't even need a DBAPI to be available.
+
+    Calls to context.execute() here emit the given string to the
+    script output.
+
+    """
+    url = config.get_main_option("sqlalchemy.url")
+    context.configure(
+        url=url,
+        target_metadata=target_metadata,
+        literal_binds=True,
+        dialect_opts={"paramstyle": "named"},
+    )
+
+    with context.begin_transaction():
+        context.run_migrations()
+
+
+def do_run_migrations(connection: Connection) -> None:
+    context.configure(connection=connection, target_metadata=target_metadata)
+
+    with context.begin_transaction():
+        context.run_migrations()
+
+
+async def run_async_migrations() -> None:
+    """In this scenario we need to create an Engine
+    and associate a connection with the context.
+
+    """
+
+    connectable = async_engine_from_config(
+        config.get_section(config.config_ini_section, {}),
+        prefix="sqlalchemy.",
+        poolclass=pool.NullPool,
+    )
+
+    async with connectable.connect() as connection:
+        await connection.run_sync(do_run_migrations)
+
+    await connectable.dispose()
+
+
+def run_migrations_online() -> None:
+    """Run migrations in 'online' mode."""
+
+    asyncio.run(run_async_migrations())
+
+
+if context.is_offline_mode():
+    run_migrations_offline()
+else:
+    run_migrations_online()