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).

docs/REWORK_PLAN.md

@@ -96,11 +96,9 @@ Memory impl: in-memory Map + EventEmitter, for tests (M3).
       memory.js             # NEW — test only
       types.js              # interface contract (JSDoc)
   server/                   # NEW
-    index.js                # Express + ws bootstrap
-    db.js                   # better-sqlite3, schema, migrations
-    turn.js                 # turn-order logic (pure, server-authoritative)
-    handlers/               # action handlers (call turn logic, persist, broadcast)
-    server.test.js          # API + WS integration tests
+    index.js                # Express + ws bootstrap, generic KV REST
+    db.js                   # better-sqlite3, docs table (KV), broadcast
+    ws-contract.test.js     # adapter vs live backend (Layer 2 test)
   shared/                   # pure logic, no I/O, importable by client + server + tests
     turn.js                 # turn logic (single source; server imports, tests import)
     types.js