feat: Phase 0 - Infrastructure and scaffolding (T-001 to T-003)

This commit establishes the foundational infrastructure for the Aegis
MITRE ATT&CK Coverage Platform.

T-001: Initialize project and Docker Compose
- Set up Docker Compose with PostgreSQL 15, MinIO, and FastAPI backend
- Create basic FastAPI application with health endpoint
- Configure persistent volumes for data storage

T-002: Configuration and database connection
- Add centralized configuration using pydantic-settings
- Implement SQLAlchemy database connection with session management
- Configure MinIO and JWT settings

T-003: Initialize Alembic for migrations
- Set up Alembic with PostgreSQL connection from settings
- Create initial empty migration
- Configure autogenerate support for future models

Also includes:
- Professional README with setup instructions
- Comprehensive .gitignore for Python/Node/Docker
- Project task plan (AegisTestPlan.md)

backend/app/database.py

@@ -0,0 +1,16 @@
+from sqlalchemy import create_engine
+from sqlalchemy.orm import sessionmaker, declarative_base
+
+from app.config import settings
+
+engine = create_engine(settings.DATABASE_URL)
+SessionLocal = sessionmaker(autocommit=False, autoflush=False, bind=engine)
+Base = declarative_base()
+
+
+def get_db():
+    db = SessionLocal()
+    try:
+        yield db
+    finally:
+        db.close()