Gamify: E-P1 cloud session plumbing + offline event queue

[fernandotonon]

Part of #796 — Gamify: editor half. Pairs with qtmesh-cloud#80.

Goal

Give the editor a reliable, offline-tolerant way to send gamification events to the account and read back stats — built on the existing cloud session, not a parallel stack.

Work

• Reuse QtMeshCloudSession / QtMeshCloudClient and the existing credential store (CloudCredentialStore) + device-flow auth. No new auth.
• Add a persistent event queue (GamificationEventQueue): append-only local store (e.g. JSON/SQLite in the app config dir) of pending events, each with a client-generated idempotency id. Survives restarts.
• Add client methods:
  • postEditorEvents(bearer, events[]) → POST /v1/events/editor (feature usage).
  • postOperationEvents(bearer, ops[]) → POST /v1/events/operations (outcomes; new cloud endpoint).
  • fetchStats(bearer) → GET /v1/me/stats → parsed {level, xp, streak, achievements, featureUsage, recentlyEarned}.
• Flush policy: batch every N minutes / on app idle / on graceful shutdown; exponential backoff on failure; drop nothing until acked. Cap queue size with FIFO eviction + a logged warning (no silent loss).
• Return newly-earned achievements from flush so E-P4 can toast them.

Acceptance criteria

• Events queued while offline/logged-out are held and flushed once authenticated + online.
• Flush is idempotent (same id never double-sent effect); backoff on 5xx; respects server retry hints.
• Queue persists across restarts and is bounded (FIFO eviction logged, never silent).
• fetchStats parses into a typed struct with unit tests.
• Zero network activity when the user is logged out.

Notes

• Keep the queue transport-agnostic so E-P2 (usage) and E-P3 (operations) just enqueue.