Add gift clause rule for free gift encounters

When enabled, in-game gift Pokemon (starters, trades, fossils) do not
count against a location's encounter limit. Both a gift encounter and
a regular encounter can coexist on the same route, in any order.

Persists encounter origin on the Encounter model so the backend can
exclude gift encounters from route-lock checks bidirectionally, and the
frontend can split them into a separate display layer that doesn't lock
the route for regular encounters.

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

backend/src/app/schemas/encounter.py

@@ -35,6 +35,7 @@ class EncounterResponse(CamelModel):
     faint_level: int | None
     death_cause: str | None
     is_shiny: bool
+    origin: str | None
     caught_at: datetime