feat: add database models and alembic setup

- SQLAlchemy async models for sources, jobs, schedules, executions, backups, settings
- Alembic migration configuration

backend/alembic.ini

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+[alembic]
+script_location = alembic
+prepend_sys_path = .
+version_path_separator = os
+sqlalchemy.url = sqlite+aiosqlite:///./backup_tool.db

backend/alembic/README

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+Generic single-database configuration.

backend/alembic/env.py

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+import asyncio
+from logging.config import fileConfig
+from sqlalchemy import pool
+from sqlalchemy.engine import Connection
+from sqlalchemy.ext.asyncio import async_engine_from_config
+from alembic import context
+from app.models import Base
+
+config = context.config
+if config.config_file_name is not None:
+    fileConfig(config.config_file_name)
+
+target_metadata = Base.metadata
+
+def run_migrations_offline() -> None:
+    url = config.get_main_option("sqlalchemy.url")
+    context.configure(
+        url=url,
+        target_metadata=target_metadata,
+        literal_binds=True,
+        dialect_opts={"paramstyle": "named"},
+    )
+    with context.begin_transaction():
+        context.run_migrations()
+
+def do_run_migrations(connection: Connection) -> None:
+    context.configure(connection=connection, target_metadata=target_metadata)
+    with context.begin_transaction():
+        context.run_migrations()
+
+async def run_async_migrations() -> None:
+    connectable = async_engine_from_config(
+        config.get_section(config.config_ini_section, {}),
+        prefix="sqlalchemy.",
+        poolclass=pool.NullPool,
+    )
+    async with connectable.connect() as connection:
+        await connection.run_sync(do_run_migrations)
+    await connectable.dispose()
+
+def run_migrations_online() -> None:
+    asyncio.run(run_async_migrations())
+
+if context.is_offline_mode():
+    run_migrations_offline()
+else:
+    run_migrations_online()

backend/alembic/script.py.mako

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+"""${message}
+
+Revision ID: ${up_revision}
+Revises: ${down_revision | comma,n}
+Create Date: ${create_date}
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+${imports if imports else ""}
+
+# revision identifiers, used by Alembic.
+revision: str = ${repr(up_revision)}
+down_revision: Union[str, Sequence[str], None] = ${repr(down_revision)}
+branch_labels: Union[str, Sequence[str], None] = ${repr(branch_labels)}
+depends_on: Union[str, Sequence[str], None] = ${repr(depends_on)}
+
+
+def upgrade() -> None:
+    """Upgrade schema."""
+    ${upgrades if upgrades else "pass"}
+
+
+def downgrade() -> None:
+    """Downgrade schema."""
+    ${downgrades if downgrades else "pass"}

backend/app/database.py

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+from sqlalchemy.ext.asyncio import create_async_engine, AsyncSession, async_sessionmaker
+from sqlalchemy.orm import declarative_base
+
+DATABASE_URL = "sqlite+aiosqlite:///./backup_tool.db"
+
+engine = create_async_engine(DATABASE_URL, echo=True)
+AsyncSessionLocal = async_sessionmaker(engine, class_=AsyncSession, expire_on_commit=False)
+
+Base = declarative_base()
+
+async def get_db():
+    async with AsyncSessionLocal() as session:
+        try:
+            yield session
+        finally:
+            await session.close()

backend/app/models.py

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+from sqlalchemy import Column, Integer, String, DateTime, Boolean, Text, ForeignKey, JSON
+from sqlalchemy.orm import relationship
+from datetime import datetime
+from .database import Base
+
+class Source(Base):
+    __tablename__ = "sources"
+    
+    id = Column(Integer, primary_key=True, index=True)
+    name = Column(String, nullable=False)
+    type = Column(String, nullable=False)  # local, ssh, database
+    config = Column(JSON, default=dict)
+    created_at = Column(DateTime, default=datetime.utcnow)
+    updated_at = Column(DateTime, default=datetime.utcnow, onupdate=datetime.utcnow)
+    
+    jobs = relationship("Job", back_populates="source", cascade="all, delete-orphan")
+
+class Job(Base):
+    __tablename__ = "jobs"
+    
+    id = Column(Integer, primary_key=True, index=True)
+    name = Column(String, nullable=False)
+    source_id = Column(Integer, ForeignKey("sources.id"), nullable=False)
+    strategy = Column(String, nullable=False, default="full")  # full, incremental
+    destination_path = Column(String, nullable=False)
+    exclude_patterns = Column(JSON, default=list)
+    enabled = Column(Boolean, default=True)
+    created_at = Column(DateTime, default=datetime.utcnow)
+    updated_at = Column(DateTime, default=datetime.utcnow, onupdate=datetime.utcnow)
+    
+    source = relationship("Source", back_populates="jobs")
+    schedule = relationship("Schedule", back_populates="job", uselist=False, cascade="all, delete-orphan")
+    executions = relationship("JobExecution", back_populates="job", cascade="all, delete-orphan")
+
+class Schedule(Base):
+    __tablename__ = "schedules"
+    
+    id = Column(Integer, primary_key=True, index=True)
+    job_id = Column(Integer, ForeignKey("jobs.id"), unique=True, nullable=False)
+    cron_expression = Column(String, nullable=False)
+    enabled = Column(Boolean, default=True)
+    created_at = Column(DateTime, default=datetime.utcnow)
+    
+    job = relationship("Job", back_populates="schedule")
+
+class JobExecution(Base):
+    __tablename__ = "job_executions"
+    
+    id = Column(Integer, primary_key=True, index=True)
+    job_id = Column(Integer, ForeignKey("jobs.id"), nullable=False)
+    status = Column(String, nullable=False, default="pending")  # pending, running, success, failed, cancelled
+    started_at = Column(DateTime, nullable=True)
+    completed_at = Column(DateTime, nullable=True)
+    bytes_processed = Column(Integer, default=0)
+    bytes_backed_up = Column(Integer, default=0)
+    error_message = Column(Text, nullable=True)
+    triggered_by = Column(String, nullable=False)  # manual, schedule
+    
+    job = relationship("Job", back_populates="executions")
+    backups = relationship("Backup", back_populates="execution", cascade="all, delete-orphan")
+
+class Backup(Base):
+    __tablename__ = "backups"
+    
+    id = Column(Integer, primary_key=True, index=True)
+    execution_id = Column(Integer, ForeignKey("job_executions.id"), nullable=False)
+    storage_path = Column(String, nullable=False)
+    size_bytes = Column(Integer, default=0)
+    checksum = Column(String, nullable=True)
+    type = Column(String, nullable=False)  # full, incremental
+    parent_backup_id = Column(Integer, ForeignKey("backups.id"), nullable=True)
+    created_at = Column(DateTime, default=datetime.utcnow)
+    
+    execution = relationship("JobExecution", back_populates="backups")
+    parent_backup = relationship("Backup", remote_side=[id])
+
+class Setting(Base):
+    __tablename__ = "settings"
+    
+    key = Column(String, primary_key=True)
+    value = Column(Text, nullable=True)
+    updated_at = Column(DateTime, default=datetime.utcnow, onupdate=datetime.utcnow)