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/seeds/inject_test_data.py

@@ -144,6 +144,7 @@ RUN_DEFS = [
 DEFAULT_RULES = {
     "duplicatesClause": True,
     "shinyClause": True,
+    "giftClause": False,
     "pinwheelClause": True,
     "levelCaps": False,
     "hardcoreMode": False,