feat(analytics): opt-in usage analytics with server ingest and settings toggle

- Server: POST /api/analytics endpoint with per-IP rate limit (5/min),
  batch validation (≤50 events, event_type regex, UUID dedup, clock check),
  INSERT OR IGNORE for idempotency, and migration 004_analytics.sql
- Client (solitaire_data): AnalyticsClient with in-memory Mutex buffer,
  UUID session_id per launch, async flush via background task
- Engine: AnalyticsPlugin records game_won, game_forfeit, game_start,
  achievement_unlocked; flushes immediately on game-end, every 60 s otherwise
- Settings UI: Privacy section with ON/OFF toggle, hidden in local-only mode
- Default: analytics_enabled = false (explicit opt-in required)

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

solitaire_engine/src/lib.rs

@@ -5,6 +5,7 @@ pub mod android_clipboard;
 pub mod assets;
 pub mod card_animation;
 pub mod achievement_plugin;
+pub mod analytics_plugin;
 pub mod animation_plugin;
 pub mod auto_complete_plugin;
 pub mod audio_plugin;
@@ -60,6 +61,7 @@ pub use theme::{
     ThemeRegistryPlugin,
 };
 pub use achievement_plugin::{AchievementPlugin, AchievementsResource, AchievementsScreen};
+pub use analytics_plugin::{AnalyticsPlugin, AnalyticsResource};
 pub use challenge_plugin::{
     challenge_progress_label, ChallengeAdvancedEvent, ChallengePlugin, CHALLENGE_UNLOCK_LEVEL,
 };