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/alembic/versions/i0d1e2f3a4b5_add_origin_to_encounters.py

@@ -0,0 +1,29 @@
+"""add origin to encounters
+
+Revision ID: i0d1e2f3a4b5
+Revises: h9c0d1e2f3a4
+Create Date: 2026-02-20 12:00:00.000000
+
+"""
+
+from collections.abc import Sequence
+
+import sqlalchemy as sa
+from alembic import op
+
+# revision identifiers, used by Alembic.
+revision: str = "i0d1e2f3a4b5"
+down_revision: str | Sequence[str] | None = "h9c0d1e2f3a4"
+branch_labels: str | Sequence[str] | None = None
+depends_on: str | Sequence[str] | None = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        "encounters",
+        sa.Column("origin", sa.String(20), nullable=True),
+    )
+
+
+def downgrade() -> None:
+    op.drop_column("encounters", "origin")