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/schemas/tree_export.py

@@ -0,0 +1,54 @@
+"""Schemas for .rfflow file export and import."""
+from datetime import datetime
+from typing import Optional, Any, Literal
+from pydantic import BaseModel, Field
+
+from app.schemas.tree import TreeType
+
+
+class FlowExportCategory(BaseModel):
+    """Category info embedded in export file."""
+    name: str
+    slug: str
+
+
+class FlowExportData(BaseModel):
+    """The flow payload inside an .rfflow file."""
+    name: str
+    description: Optional[str] = None
+    tree_type: TreeType
+    version: int = 1
+    author_name: Optional[str] = None
+    category: Optional[FlowExportCategory] = None
+    tags: list[str] = []
+    tree_structure: dict[str, Any]
+    intake_form: Optional[list[dict[str, Any]]] = None
+
+
+class FlowExportEnvelope(BaseModel):
+    """Top-level .rfflow file structure."""
+    rfflow_version: str = "1.0"
+    exported_at: datetime
+    source_app: str = "ResolutionFlow"
+    format: Literal["json", "xml"] = "json"
+    flow: FlowExportData
+
+
+class FlowImportRequest(BaseModel):
+    """What the frontend sends after parsing a .rfflow file."""
+    rfflow_version: str = Field(..., description="Must be '1.0'")
+    exported_at: datetime
+    source_app: str = "ResolutionFlow"
+    format: Literal["json", "xml"] = "json"
+    flow: FlowExportData
+
+
+class FlowImportResponse(BaseModel):
+    """Response after importing a flow."""
+    tree_id: str
+    name: str
+    tree_type: str
+    status: str = "draft"
+    category_created: bool = False
+    tags_created: list[str] = []
+    validation_warnings: list[str] = []