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/models/encounter.py

@@ -24,6 +24,7 @@ class Encounter(Base):
     is_shiny: Mapped[bool] = mapped_column(
         Boolean, default=False, server_default=text("false")
     )
+    origin: Mapped[str | None] = mapped_column(String(20))
     caught_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True), server_default=func.now()
     )