feat(database): setup alembic and create scenarios table

- Install alembic and asyncpg for database migrations
- Configure alembic for async SQLAlchemy 2.0
- Create initial migration for scenarios table:
  * UUID primary key with auto-generation
  * Status enum (draft, running, completed, archived)
  * JSONB tags with GIN index
  * Timestamps with auto-update trigger
  * Check constraints for name/region validation
  * Indexes on status, region, created_at
- Test database connection and migration

Task: DB-001, DB-002 complete

pyproject.toml

@@ -6,6 +6,8 @@ readme = "README.md"
 requires-python = ">=3.11"
 
 dependencies = [
+    "alembic>=1.18.4",
+    "asyncpg>=0.31.0",
     "fastapi>=0.110.0",
     "pydantic>=2.7.0",
     "tiktoken>=0.6.0",