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/app/models/tree.py

@@ -154,6 +154,13 @@ class Tree(Base):
         comment="Fork depth: 0 = original, 1 = direct fork, 2 = fork of fork, etc."
     )
 
+    # Import provenance
+    import_metadata: Mapped[Optional[dict[str, Any]]] = mapped_column(
+        JSONB,
+        nullable=True,
+        comment="Provenance metadata from .rfflow file import"
+    )
+
     # Relationships
     author: Mapped[Optional["User"]] = relationship("User", foreign_keys=[author_id], back_populates="trees")
     team: Mapped[Optional["Team"]] = relationship("Team", back_populates="trees")