feat(pathfinder): add per-room underpass setting via room settings UI

Add allow_underpass as a per-room boolean setting that controls whether
avatars can walk under elevated furniture. When disabled (default), the
room behaves normally with blocking items. When enabled, items elevated
above the UNDERPASS_HEIGHT threshold allow walking underneath.

Changes:
- Room: add allowUnderpass field with DB load/save
- RoomTileManager: make all 3 underpass checks conditional on room setting
- RoomItemManager: getWalkableItemAt() falls back when underpass disabled
- RoomSettingsComposer/SaveEvent: send/receive the flag in room settings packet
- SQL migration: add allow_underpass column to rooms table

Co-Authored-By: medievalshell <medievalshell@users.noreply.github.com>

Database Updates/17032026_allow_underpass.sql

@@ -0,0 +1 @@
+ALTER TABLE `rooms` ADD COLUMN `allow_underpass` ENUM('0','1') NOT NULL DEFAULT '0' AFTER `move_diagonally`;