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>

frontend/src/components/EncounterModal.tsx

@@ -26,6 +26,7 @@ interface EncounterModalProps {
     nickname?: string | undefined
     status: EncounterStatus
     catchLevel?: number | undefined
+    origin?: string | undefined
   }) => void
   onUpdate?:
     | ((data: {
@@ -291,6 +292,7 @@ export function EncounterModal({
         nickname: nickname || undefined,
         status,
         catchLevel: catchLevel ? Number(catchLevel) : undefined,
+        origin: SPECIAL_METHODS.includes(selectedPokemon.encounterMethod) ? 'gift' : undefined,
       })
     }
   }