fix: remove unused imports and variables from server files to resolve build errors.

docs/development/devlog/2025-12-19-011500_fix_build_unused_vars.md

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+# Fix Build Unused Variables
+
+## Objective
+Fix Typescript errors preventing `npm run build` execution in the Docker container.
+The errors were `TS6133` (unused variables) in:
+- `server/game/RulesEngine.ts`
+- `server/managers/PersistenceManager.ts`
+- `server/services/CardService.ts`
+
+## Changes
+1. **RulesEngine.ts**:
+   - Removed unused imports: `PlayerState`, `StackObject`.
+   - Renamed unused parameter `playerId` to `_playerId` in `cleanupStep`.
+   - (Also fixed an accidental comment injection during the process).
+
+2. **PersistenceManager.ts**:
+   - Removed unused `__dirname` and `__filename` definitions.
+   - Removed unused `fileURLToPath` import.
+
+3. **CardService.ts**:
+   - Removed unused `fs` import.
+
+## Verification
+- Ran `npx tsc --noEmit` in `src` directory. Result: Exit code 0 (Success).