M2: replace shape-specific backend with generic KV doc store (firebase mirror)

Backend was endpoint-mapper (POST /api/campaigns ignores path id, etc). Adapter
translation layer brittle, untested, lost doc identity. Generic contract (Layer 2)
test caught 15 bugs immediately.

Rewrite to firebase-mirror KV model:
- server/db.js: docs table (path PK, parent, data JSON). getDoc/setDoc/updateDoc/
  deleteDoc/getCollection/batchWrite. parent = path prefix for collection queries.
- server/index.js: generic REST (GET/PUT/PATCH/DELETE /api/doc, GET /api/collection,
  POST /api/collection addDoc, POST /api/batch). WS subscribe by path (doc|collection),
  broadcast to doc subs at changed path + collection subs at parent path.
- src/storage/ws.js: thin passthrough adapter. norm() strips firebase prefix.
  initial value via REST (independent of WS connect), subsequent changes via WS.
- shared/turn.js kept (M4 use). server/handlers.js + server.test.js removed (logic
  now in App, backend is dumb KV).
- src/storage/contract.js: flush() bumped to 50ms (WS roundtrip > setTimeout(0)).

Layer 2 test (server/ws-contract.test.js): spins fresh backend per test, runs same
storage contract spec against createWsStorage. Catches adapter translation bugs
that firebase-mock Layer 1 tests cannot.

nanoid v5 ESM breaks jest CJS -> replaced with crypto.randomUUID (Node builtin).

Tests: 114 green (39 shared + 19 ws-contract + 56 frontend).

src/storage/contract.js

@@ -191,9 +191,10 @@ function runStorageContract(name, factory) {
   });
 }
 
-// microtask flush so async subscribers settle.
+// flush so async subscribers settle. WS roundtrip needs real delay (network),
+// memory fires near-instant. 50ms covers localhost WS comfortably.
 function flush() {
-  return new Promise((resolve) => setTimeout(resolve, 0));
+  return new Promise((resolve) => setTimeout(resolve, 50));
 }
 
 module.exports = { runStorageContract, flush };