fix(broker): shareContext stable upsert key + createStream atomic upsert

- shareContext: adds optional memberId param; when provided, upserts on
  (meshId, memberId) instead of (meshId, presenceId) — prevents stale
  context rows accumulating on every reconnect. Falls back to presenceId
  for legacy/anonymous connections. Also refreshes presenceId on update
  so it stays current.
- schema: adds member_id column + unique index context_mesh_member_idx
  on mesh.context table; new migration 0013_context-stable-member-key.sql.
- index.ts call site updated to pass conn.memberId as the stable key.
- createStream: replaces SELECT-then-INSERT TOCTOU race with atomic
  INSERT ... ON CONFLICT DO NOTHING RETURNING, followed by SELECT on miss.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/db/migrations/0013_context-stable-member-key.sql

@@ -0,0 +1,3 @@
+ALTER TABLE "mesh"."context" ADD COLUMN "member_id" text;--> statement-breakpoint
+ALTER TABLE "mesh"."context" ADD CONSTRAINT "context_member_id_member_id_fk" FOREIGN KEY ("member_id") REFERENCES "mesh"."member"("id") ON DELETE cascade ON UPDATE cascade;--> statement-breakpoint
+CREATE UNIQUE INDEX "context_mesh_member_idx" ON "mesh"."context" ("mesh_id","member_id");