Add SQLAlchemy 2.0 with Repository Pattern

[Presmanes3]

Problem

src/climatevision/db.py (551 lines) contains all database access with raw SQL strings and standard sqlite3. This has several issues:

• No type safety in queries
• No reusable ORM models
• No schema migrations
• SQLite in production (no PostgreSQL support)
• Boilerplate CRUD code
• Hard to test with in-memory database

Proposed solution

Implement SQLAlchemy 2.0 async with Repository Pattern:

src/climatevision/
└── database/
   ├── __init__.py
   ├── models.py            # SQLAlchemy ORM models (Run, Organization, Alert, etc.)
   ├── connection.py        # Async engine + session factory
   ├── repositories/
   │   ├── __init__.py
   │   ├── base.py         # Abstract BaseRepository[T]
   │   ├── run_repository.py
   │   ├── org_repository.py
   │   └── alert_repository.py
   └── migrations/        # Alembic migrations

Tasks

• [ ] Add SQLAlchemy and alembic to dependencies
• [ ] Create ORM models for all tables
• [ ] Implement generic BaseRepository
• [ ] Implement specific repositories
• [ ] Configure Alembic for migrations
• [ ] Replace db.py references in API layer
• [ ] Update tests with in-memory database fixtures
• [ ] Support PostgreSQL via DATABASE_URL env var