docs(project): add README, CI workflow, migration guide, and fix asset docs

- README.md: player-facing install, controls, features, and test instructions
- .github/workflows/ci.yml: clippy + headless tests + release build on push/PR
- solitaire_server/migrations/README.md: naming convention and workflow for
  adding future schema migrations
- ARCHITECTURE.md §14: rewrite Asset Pipeline to reflect procedural rendering
  (no image files used; audio only, embedded via include_bytes!)
- ARCHITECTURE.md §2 / §13: fix workspace structure and audio file listing
- CLAUDE.md: clarify asset embedding rule (audio only; visuals are procedural)
- server_tests.rs: add auth_rate_limit_returns_429_on_11th_request test using
  build_router() (rate limiting ON) to verify the GovernorLayer is wired correctly

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

CLAUDE.md

@@ -47,7 +47,7 @@ cargo clippy -p solitaire_core -- -D warnings
 
 - `solitaire_core` and `solitaire_sync` must never gain Bevy or network dependencies.
 - No `unwrap()` or `panic!()` in game logic. All state transitions return `Result<_, MoveError>`.
-- Assets are embedded at compile time using `include_bytes!()`. No runtime asset loading via `AssetServer`.
+- Audio assets are embedded at compile time using `include_bytes!()` in `audio_plugin.rs`. Cards and backgrounds are rendered procedurally (colored `Sprite` entities + text) — no image files are used and no `AssetServer` is needed.
 - Atomic file writes only: write to `filename.json.tmp`, then `rename()`.
 - Passwords and tokens are stored in the OS keychain via the `keyring` crate — never in plaintext files or logs.
 - Sync runs on `AsyncComputeTaskPool` — never block the Bevy main thread.