feat: implement background task architecture for code quality scans

- Add status fields to ArchitectureScan model (status, started_at,
  completed_at, error_message, progress_message)
- Create database migration for new status fields
- Create background task function execute_code_quality_scan()
- Update API to return 202 with job IDs and support polling
- Add code quality scans to unified BackgroundTasksService
- Integrate scans into background tasks API and page
- Implement frontend polling with 3-second interval
- Add progress banner showing scan status
- Users can navigate away while scans run in background
- Document the implementation in architecture docs

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

alembic/versions/g5b6c7d8e9f0_add_scan_status_fields.py

@@ -0,0 +1,82 @@
+"""add_scan_status_fields
+
+Add background task status fields to architecture_scans table
+for harmonized background task architecture.
+
+Revision ID: g5b6c7d8e9f0
+Revises: f4a5b6c7d8e9
+Create Date: 2024-12-21
+
+"""
+
+from collections.abc import Sequence
+
+import sqlalchemy as sa
+from alembic import op
+
+# revision identifiers, used by Alembic.
+revision: str = "g5b6c7d8e9f0"
+down_revision: str | None = "f4a5b6c7d8e9"
+branch_labels: str | Sequence[str] | None = None
+depends_on: str | Sequence[str] | None = None
+
+
+def upgrade() -> None:
+    # Add status field with default 'completed' for existing records
+    # New records will use 'pending' as default
+    op.add_column(
+        "architecture_scans",
+        sa.Column(
+            "status",
+            sa.String(length=30),
+            nullable=False,
+            server_default="completed",  # Existing scans are already completed
+        ),
+    )
+    op.create_index(
+        op.f("ix_architecture_scans_status"), "architecture_scans", ["status"]
+    )
+
+    # Add started_at - for existing records, use timestamp as started_at
+    op.add_column(
+        "architecture_scans",
+        sa.Column("started_at", sa.DateTime(timezone=True), nullable=True),
+    )
+
+    # Add completed_at - for existing records, use timestamp + duration as completed_at
+    op.add_column(
+        "architecture_scans",
+        sa.Column("completed_at", sa.DateTime(timezone=True), nullable=True),
+    )
+
+    # Add error_message for failed scans
+    op.add_column(
+        "architecture_scans",
+        sa.Column("error_message", sa.Text(), nullable=True),
+    )
+
+    # Add progress_message for showing current step
+    op.add_column(
+        "architecture_scans",
+        sa.Column("progress_message", sa.String(length=255), nullable=True),
+    )
+
+    # Update existing records to have proper started_at and completed_at
+    # This is done via raw SQL for efficiency
+    op.execute(
+        """
+        UPDATE architecture_scans
+        SET started_at = timestamp,
+            completed_at = datetime(timestamp, '+' || CAST(duration_seconds AS TEXT) || ' seconds')
+        WHERE started_at IS NULL
+        """
+    )
+
+
+def downgrade() -> None:
+    op.drop_index(op.f("ix_architecture_scans_status"), table_name="architecture_scans")
+    op.drop_column("architecture_scans", "progress_message")
+    op.drop_column("architecture_scans", "error_message")
+    op.drop_column("architecture_scans", "completed_at")
+    op.drop_column("architecture_scans", "started_at")
+    op.drop_column("architecture_scans", "status")