feat: add GPU/CUDA acceleration support (:cuda Docker tag)

Add a :cuda Docker image tag that auto-detects NVIDIA GPU at runtime
and falls back gracefully to CPU. Same pattern as Immich.

- New gpu.py shared utility for cached CUDA detection
- Background removal (rembg): pass CUDAExecutionProvider to ONNX Runtime
- Upscaling (Real-ESRGAN): use CUDA device + FP16 when GPU available
- OCR (PaddleOCR): enable use_gpu when CUDA detected
- Dispatcher reports GPU status at startup via readiness signal
- Admin health endpoint exposes GPU availability
- Dockerfile uses ARG GPU=false with conditional NVIDIA CUDA base image
- docker-compose.gpu.yml override for GPU users
- CI/CD workflows build and publish :cuda tag (amd64 only)

Three tags: :latest (CPU), :lite (no AI), :cuda (GPU with CPU fallback)

apps/api/src/index.ts

@@ -1,5 +1,6 @@
 import cors from "@fastify/cors";
 import rateLimit from "@fastify/rate-limit";
+import { isGpuAvailable } from "@stirling-image/ai";
 import { APP_VERSION } from "@stirling-image/shared";
 import Fastify from "fastify";
 import { env } from "./config.js";
@@ -132,7 +133,7 @@ app.get("/api/v1/admin/health", async (request, reply) => {
     storage: { mode: env.STORAGE_MODE, available: "N/A" },
     database: dbOk ? "ok" : "error",
     queue: { active: 0, pending: 0 },
-    ai: {},
+    ai: { gpu: isGpuAvailable() },
   };
 });