feat: unified Docker image with GPU auto-detection (#37)

Merge CPU, CUDA, and lite Docker images into a single unified image.
One tag (latest) works on all platforms: amd64 (NVIDIA CUDA) and arm64 (CPU).
GPU auto-detected at runtime. All ML models and packages baked in.

Key changes:
- Platform-conditional Dockerfile (nvidia/cuda on amd64, node on arm64)
- tini as PID 1 for proper signal handling
- Fix FILES_STORAGE_PATH data loss bug
- Fix RealESRGAN upscaler (was broken, always fell back to Lanczos)
- Fix PaddleOCR language codes and stdout corruption
- Simplified CI/CD (single build, single tag)
- Expanded model pre-download with verification
- Shutdown timeout, improved health endpoint
- Remove unused lama-cleaner

docker/docker-compose.gpu.yml

@@ -1,17 +0,0 @@
-# GPU override - use with:
-#   docker compose -f docker/docker-compose.yml -f docker/docker-compose.gpu.yml up
-services:
-  stirling-image:
-    build:
-      context: ..
-      dockerfile: docker/Dockerfile
-      args:
-        GPU: "true"
-    image: stirlingimage/stirling-image:cuda
-    deploy:
-      resources:
-        reservations:
-          devices:
-            - driver: nvidia
-              count: 1
-              capabilities: [gpu]