feat: add backup engine with full/incremental support

- BackupEngine class executes jobs and creates backups
- Automatic fallback from incremental to full if no parent exists
- SHA-256 checksum calculation for integrity
- Tests for full backup and incremental fallback

backend/tests/test_engine.py

@@ -0,0 +1,71 @@
+import pytest
+import tempfile
+import os
+from pathlib import Path
+from sqlalchemy.ext.asyncio import AsyncSession
+from app.models import Source, Job, JobExecution
+from backup.engine import BackupEngine
+
+@pytest.fixture
+async def test_source(db: AsyncSession):
+    with tempfile.TemporaryDirectory() as tmpdir:
+        # Create test files
+        (Path(tmpdir) / "test.txt").write_text("Hello, World!")
+        (Path(tmpdir) / "subdir").mkdir()
+        (Path(tmpdir) / "subdir" / "nested.txt").write_text("Nested content")
+        
+        source = Source(
+            name="Test Source",
+            type="local",
+            config={"path": tmpdir}
+        )
+        db.add(source)
+        await db.commit()
+        await db.refresh(source)
+        yield source
+
+@pytest.fixture
+async def test_job(db: AsyncSession, test_source):
+    with tempfile.TemporaryDirectory() as tmpdir:
+        job = Job(
+            name="Test Job",
+            source_id=test_source.id,
+            strategy="full",
+            destination_path=tmpdir
+        )
+        db.add(job)
+        await db.commit()
+        await db.refresh(job)
+        yield job
+
+@pytest.mark.asyncio
+async def test_execute_full_backup(db: AsyncSession, test_job):
+    engine = BackupEngine(db)
+    execution = await engine.execute_job(test_job.id, triggered_by="manual")
+    
+    assert execution.status == "success"
+    assert execution.bytes_processed > 0
+    assert execution.bytes_backed_up > 0
+    assert execution.triggered_by == "manual"
+    
+    # Verify backup was created
+    assert len(test_job.executions) == 1
+    backup = test_job.executions[0].backups[0]
+    assert backup.type == "full"
+    assert backup.checksum is not None
+    assert os.path.exists(backup.storage_path)
+
+@pytest.mark.asyncio
+async def test_execute_incremental_without_full(db: AsyncSession, test_job):
+    # Set job to incremental but no full backup exists
+    test_job.strategy = "incremental"
+    await db.commit()
+    
+    engine = BackupEngine(db)
+    execution = await engine.execute_job(test_job.id)
+    
+    # Should fall back to full backup
+    assert execution.status == "success"
+    backup = execution.backups[0]
+    assert backup.type == "full"
+    assert backup.parent_backup_id is None