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>

Dockerfile

@@ -29,15 +29,7 @@ COPY . .
 ENV NEXT_TELEMETRY_DISABLED=1
 ENV NODE_ENV=production
 
-# Accept build args for environment variables
-ARG NEXT_PUBLIC_API_URL
-ARG NEXT_PUBLIC_APP_URL
-
-# Set environment variables for Next.js build
-ENV NEXT_PUBLIC_API_URL=${NEXT_PUBLIC_API_URL}
-ENV NEXT_PUBLIC_APP_URL=${NEXT_PUBLIC_APP_URL}
-
-# Build the application
+# Build the application (no API URL needed at build time - uses proxy)
 RUN pnpm build
 
 # Stage 3: Runner