feat: implement runtime configuration with API proxy pattern

Replace build-time NEXT_PUBLIC_* environment variables with server-side
runtime configuration. This allows changing the Pastel API URL without
rebuilding the Docker image.

**Changes:**
- Add Next.js API proxy route at /api/pastel/[...path] for server-side proxying
- Update API client to use proxy endpoint instead of direct API URL
- Replace NEXT_PUBLIC_API_URL with server-side PASTEL_API_URL
- Remove build arguments from Dockerfile (no longer needed)
- Simplify docker-compose.yml to use runtime environment variables only
- Update all .env files to reflect new configuration approach
- Add comprehensive DOCKER.md documentation

**Benefits:**
- No rebuild required to change API URL
- Same image works across all environments (dev/staging/prod)
- Better security (API URL not exposed in client bundle)
- Simpler deployment and configuration management

**Migration:**
Old: NEXT_PUBLIC_API_URL (build-time, embedded in bundle)
New: PASTEL_API_URL (runtime, read by server proxy)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

lib/api/client.ts

@@ -35,14 +35,18 @@ export class PastelAPIClient {
   private baseURL: string;
 
   constructor(baseURL?: string) {
-    this.baseURL = baseURL || process.env.NEXT_PUBLIC_API_URL || 'http://localhost:3000';
+    // Use the Next.js API proxy route for runtime configuration
+    // This allows changing the backend API URL without rebuilding
+    this.baseURL = baseURL || '/api/pastel';
   }
 
   private async request<T>(
     endpoint: string,
     options?: RequestInit
   ): Promise<ApiResponse<T>> {
-    const url = `${this.baseURL}/api/v1${endpoint}`;
+    // Endpoint already includes /api/v1 prefix on backend,
+    // but our proxy route expects paths after /api/v1/
+    const url = `${this.baseURL}${endpoint}`;
 
     try {
       const response = await fetch(url, {