Phase 4: web markers and JourneyMap/Xaero waypoint chat sharing

Linked accounts can place 2D markers on the map (y auto-derived from the
chunk store's heightmap); anonymous visitors keep a localStorage-only
list via the same height lookup. Sharing a marker forwards a structured
payload to the mod over its WS connection, which builds the actual
chat text (see MCMapper-Mod for the JourneyMap/Xaero formatting).

Built test-first per the project's TDD workflow.

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

api/drizzle/0003_markers.sql

@@ -0,0 +1,16 @@
+ALTER TABLE "servers" ADD COLUMN IF NOT EXISTS "waypoint_format" text NOT NULL DEFAULT 'journeymap';
+
+CREATE TABLE IF NOT EXISTS "markers" (
+	"id" uuid PRIMARY KEY DEFAULT gen_random_uuid() NOT NULL,
+	"account_id" uuid NOT NULL REFERENCES "accounts"("id") ON DELETE CASCADE,
+	"server_id" uuid NOT NULL REFERENCES "servers"("id") ON DELETE CASCADE,
+	"dimension" integer NOT NULL,
+	"x" integer NOT NULL,
+	"y" integer NOT NULL,
+	"z" integer NOT NULL,
+	"name" text NOT NULL,
+	"color" text NOT NULL,
+	"created_at" timestamptz NOT NULL DEFAULT now()
+);
+
+CREATE INDEX IF NOT EXISTS "markers_account_server_idx" ON "markers" ("account_id", "server_id");