feat: add flow export/import backend (migration, endpoints, schemas)

Add .rfflow file export/import support:
- Migration 050: import_metadata JSONB column on trees
- GET /trees/{id}/export?format=json|xml endpoint
- POST /trees/import endpoint (creates draft, resolves categories/tags)
- FlowExportEnvelope, FlowImportRequest/Response schemas
- import_metadata field on TreeResponse

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/alembic/versions/050_add_import_metadata_to_trees.py

@@ -0,0 +1,23 @@
+"""Add import_metadata JSONB column to trees table.
+
+Revision ID: 050
+Revises: 049
+Create Date: 2026-03-05
+"""
+from alembic import op
+import sqlalchemy as sa
+from sqlalchemy.dialects.postgresql import JSONB
+
+# revision identifiers
+revision = '050'
+down_revision = '049'
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.add_column('trees', sa.Column('import_metadata', JSONB, nullable=True))
+
+
+def downgrade() -> None:
+    op.drop_column('trees', 'import_metadata')