[FEATURE] Database Session & Models — SQLAlchemy tables for accounts, transactions, alerts, loyalty

[gelluisaac]

Description

Define the SQLAlchemy ORM models and database session management for the API backend.
The existing Alembic migrations should be extended or new migration created.

Models to Create

• Account — Stellar account info (id, public_key, first_seen, last_active, balance, etc.)
• Transaction — Blockchain transactions (hash, ledger, source, destination, amount, asset, fee, timestamp)
• FraudAlert — Anomaly detection results (account_id, pattern, risk_score, description, detected_at)
• LoyaltyPoints — Points balance per account (account_id, balance, tier, multiplier)
• PointsTransaction — Earn/redeem/adjust records (account_id, type, points, source, timestamp)
• ModelRegistry — Registered model versions (name, version, path, metrics, created_at)

Session Management

• Create async SQLAlchemy engine and session factory
• Provide a get_db dependency for FastAPI
• Support both sync and async endpoints

Acceptance Criteria

• All models are defined in api/models/ directory
• Tables are created by running alembic upgrade head
• Session management works with FastAPI dependency injection
• Basic CRUD queries work in pytest against a test database

Labels

enhancement, api, database