perf: optimize drawings endpoint with caching and lazy loading

- Add 5s in-memory cache for /drawings responses with automatic cleanup
- Split Drawing/DrawingSummary types for efficient data fetching
- Implement lazy loading of drawing data in DrawingCard component
- Add configurable DRAWINGS_CACHE_TTL_MS and RATE_LIMIT_MAX_REQUESTS env vars
- Prevent memory leaks with periodic cleanup of cache and rate limit maps
- Add loading states and better UX for export operations
- Improve JSON parsing with error handling for malformed stored data

Benchmark results (100 drawings, cached):
- Avg latency: 6.94ms (p50: 4ms, p97.5: 8ms)
- Avg throughput: 668 req/s (peak: 1,023)
- 3k requests in 5s with 0 errors

Update .gitignore to exclude generated files, env files, and build artifacts

frontend/src/types/index.ts

@@ -1,13 +1,17 @@
-export interface Drawing {
+export interface DrawingSummary {
   id: string;
   name: string;
-  elements: any[];
-  appState: any;
-  files: Record<string, any> | null;
   collectionId: string | null;
   updatedAt: number;
   createdAt: number;
-  preview?: string;
+  version: number;
+  preview?: string | null;
+}
+
+export interface Drawing extends DrawingSummary {
+  elements: any[];
+  appState: any;
+  files: Record<string, any> | null;
 }
 
 export interface Collection {