refactor(backend): refactor project, add database migrations support

backend/app/core/.keep

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+# Konfigurace a utility
+

backend/app/core/base.py

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+from sqlalchemy.ext.declarative import DeclarativeMeta, declarative_base
+
+Base: DeclarativeMeta = declarative_base()
+

backend/app/core/db.py

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+import os
+from sqlalchemy.ext.asyncio import create_async_engine, async_sessionmaker
+from app.core.base import Base  # Import Base z nového souboru
+
+DATABASE_URL = os.getenv("DATABASE_URL")
+if not DATABASE_URL:
+    mariadb_host = os.getenv("MARIADB_HOST", "localhost")
+    mariadb_port = os.getenv("MARIADB_PORT", "3306")
+    mariadb_db = os.getenv("MARIADB_DB", "group_project")
+    mariadb_user = os.getenv("MARIADB_USER", "root")
+    mariadb_password = os.getenv("MARIADB_PASSWORD", "strongpassword")
+    if mariadb_host and mariadb_db and mariadb_user and mariadb_password:
+        DATABASE_URL = f"mysql+asyncmy://{mariadb_user}:{mariadb_password}@{mariadb_host}:{mariadb_port}/{mariadb_db}"
+    else:
+        raise Exception("Only MariaDB is supported. Please set the DATABASE_URL environment variable.")
+
+# Explicitní import všech modelů, aby byly registrovány v Base.metadata
+from app.models.user import User
+from app.models.transaction import Transaction
+
+from app.models.user import User
+
+# Pokud máš další modely, importuj je zde:
+# from app.models.other_model import OtherModel
+
+ssl_enabled = os.getenv("MARIADB_HOST", "localhost") != "localhost"
+connect_args = {"ssl": {"ssl": True}} if ssl_enabled else {}
+
+engine = create_async_engine(
+    DATABASE_URL,
+    pool_pre_ping=True,
+    echo=os.getenv("SQL_ECHO", "0") == "1",
+    connect_args=connect_args,
+)
+async_session_maker = async_sessionmaker(engine, expire_on_commit=False)

backend/app/core/queue.py

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+import json
+import os
+from typing import Any, Dict
+import asyncio
+
+RABBITMQ_URL = os.getenv("RABBITMQ_URL") or (
+    f"amqp://{os.getenv('RABBITMQ_USERNAME', 'user')}:"
+    f"{os.getenv('RABBITMQ_PASSWORD', 'bitnami123')}@"
+    f"{os.getenv('RABBITMQ_HOST', 'localhost')}:"
+    f"{os.getenv('RABBITMQ_PORT', '5672')}"
+)
+QUEUE_NAME = os.getenv("MAIL_QUEUE", "mail_queue")
+
+async def _publish_async(message: Dict[str, Any]) -> None:
+    import aio_pika
+    connection = await aio_pika.connect_robust(RABBITMQ_URL)
+    try:
+        channel = await connection.channel()
+        await channel.declare_queue(QUEUE_NAME, durable=True)
+        body = json.dumps(message).encode("utf-8")
+        await channel.default_exchange.publish(
+            aio_pika.Message(body=body, delivery_mode=aio_pika.DeliveryMode.PERSISTENT),
+            routing_key=QUEUE_NAME,
+        )
+    finally:
+        await connection.close()
+
+def enqueue_email(to: str, subject: str, body: str) -> None:
+    message = {"type": "email", "to": to, "subject": subject, "body": body}
+    try:
+        loop = asyncio.get_running_loop()
+        loop.create_task(_publish_async(message))
+    except RuntimeError:
+        asyncio.run(_publish_async(message))
+# ...existing code...
+