Handle Nincada split evolution (Ninjask + Shedinja)

When evolving Nincada, a confirmation prompt now offers to also add
Shedinja as a new encounter on the same route. The Shedinja encounter
uses a "shed_evolution" origin to bypass route-locking.

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

backend/src/app/schemas/encounter.py

@@ -12,6 +12,7 @@ class EncounterCreate(CamelModel):
     status: str
     catch_level: int | None = None
     is_shiny: bool = False
+    origin: str | None = None
 
 
 class EncounterUpdate(CamelModel):