fix: persist edge waypoints in backend

Add waypoints JSON column to edges table, include it in all edge
schemas (EdgeBase, EdgeUpdate, canvas CanvasEdge) and add the
idempotent ALTER TABLE migration so existing databases are upgraded
on next startup.

backend/app/db/database.py

@@ -61,6 +61,8 @@ async def init_db() -> None:
             await conn.exec_driver_sql("ALTER TABLE nodes ADD COLUMN bottom_handles INTEGER NOT NULL DEFAULT 1")
         with suppress(OperationalError):
             await conn.exec_driver_sql("ALTER TABLE pending_devices ADD COLUMN discovery_source TEXT")
+        with suppress(OperationalError):
+            await conn.exec_driver_sql("ALTER TABLE edges ADD COLUMN waypoints JSON")
         # Migrate animated column from boolean (0/1) to string ('none'/'snake')
         with suppress(OperationalError):
             await conn.exec_driver_sql("UPDATE edges SET animated = 'snake' WHERE animated = '1' OR animated = 1")

backend/app/db/models.py

@@ -69,6 +69,7 @@ class Edge(Base):
     animated: Mapped[str] = mapped_column(String, nullable=False, default='none')
     source_handle: Mapped[str | None] = mapped_column(String)
     target_handle: Mapped[str | None] = mapped_column(String)
+    waypoints: Mapped[list | None] = mapped_column(JSON, nullable=True)
     created_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), default=_now)
 
 

backend/app/schemas/canvas.py

@@ -49,6 +49,7 @@ class EdgeSave(BaseModel):
     animated: str = 'none'
     source_handle: str | None = None
     target_handle: str | None = None
+    waypoints: list | None = None
 
     @field_validator('animated', mode='before')
     @classmethod

backend/app/schemas/edges.py

@@ -17,6 +17,7 @@ class EdgeBase(BaseModel):
     animated: str = 'none'
     source_handle: str | None = None
     target_handle: str | None = None
+    waypoints: list | None = None
 
     @field_validator('animated', mode='before')
     @classmethod
@@ -38,6 +39,7 @@ class EdgeUpdate(BaseModel):
     animated: str | None = None
     source_handle: str | None = None
     target_handle: str | None = None
+    waypoints: list | None = None
 
     @field_validator('animated', mode='before')
     @classmethod