Markers: editable, manual x/y/z popup placement, drop heightmap auto-derive

Markers now carry a caller-supplied x/y/z instead of an auto-resolved
terrain height: clicking the map opens a real Leaflet popup pre-filled
with the clicked x/z, a default y of 60, and a random color, all
editable before saving. The same popup now also opens for editing an
existing marker (new updateMarker/PATCH /api/markers/:markerId, TDD'd
in markers.test.ts/index.test.ts). Removes the now-unused
resolveHeight/GET /api/height machinery. Linked-account markers
already synced server-side via the markers table, so cross-device
sync falls out of the existing loadMarkers()-on-init flow.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015tKdPZt78zbPUZMXWzKEKt

api/src/db/schema.ts

@@ -60,9 +60,9 @@ export const chatMessages = pgTable("chat_messages", {
 
 // A web-placed marker, owned by a linked account (anonymous visitors keep their marker list in
 // browser localStorage only — see the plan's "Feature: web markers..." section — so there's
-// nothing to persist here for them). Placement is 2D-only (x/z picked on the Leaflet map); `y` is
-// resolved once at creation time from `chunkColumns`' heightmap and stored, not recomputed later,
-// so a marker doesn't silently move if the terrain above it changes afterward.
+// nothing to persist here for them). x/y/z are all set directly by the player in the web UI's
+// click-to-place/edit popup (x/z pre-filled from the click, y defaulting to 60), not derived
+// server-side, and can be changed later via the same popup (see markers.ts's updateMarker).
 export const markers = pgTable("markers", {
   id: uuid("id").defaultRandom().primaryKey(),
   accountId: uuid("account_id")