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>

solitaire_server/migrations/README.md

@@ -0,0 +1,38 @@
+# Database Migrations
+
+Migrations are run automatically at server startup via `sqlx::migrate!("./migrations")`.
+
+## Naming convention
+
+```
+NNN_description.sql
+```
+
+- `NNN` — zero-padded three-digit sequence number (`001`, `002`, …)
+- `description` — snake_case description of what the migration does
+
+Examples:
+```
+001_initial.sql
+002_add_user_display_name.sql
+003_weekly_goals_table.sql
+```
+
+`sqlx` tracks which migrations have run in the `_sqlx_migrations` table and only applies new ones. Never edit or delete an existing migration file after it has been applied to any database — add a new migration instead.
+
+## Adding a migration
+
+1. Create `migrations/NNN_description.sql` where `NNN` is the next available number.
+2. Write idempotent SQL (`CREATE TABLE IF NOT EXISTS`, `ALTER TABLE … ADD COLUMN IF NOT EXISTS`, etc.) where possible.
+3. Update the sqlx offline query cache so the server builds without a live DB:
+   ```bash
+   export DATABASE_URL=sqlite://solitaire.db
+   sqlx database create
+   sqlx migrate run --source solitaire_server/migrations
+   cargo sqlx prepare --workspace
+   ```
+4. Commit both the migration file and the updated `.sqlx/` query cache together.
+
+## Current schema
+
+See `001_initial.sql` for the full initial schema: `users`, `sync_state`, `daily_challenges`, `leaderboard`.