feat: GPU acceleration support for ONNX Runtime inference

Centralize execution provider selection in providers.py with
auto-detection and graceful fallback. All ONNX sessions (embedder,
detector, face processor, recognizer) now use the configured providers.

- New VISION_EXECUTION_PROVIDERS env var: "auto" for GPU auto-detect,
  or explicit "CUDAExecutionProvider,CPUExecutionProvider"
- Provider priority: CUDA > ROCm > OpenVINO > CPU (when set to "auto")
- docker-compose.yml includes commented-out NVIDIA GPU deploy section
- Supports onnxruntime-gpu as a drop-in replacement for onnxruntime

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/config.py

@@ -66,6 +66,11 @@ class VisionSettings(BaseModel):
     enabled: bool = True
     backend: str = "onnx"           # "onnx" | "rocm" (future)
     models_dir: str = "/data/models"
+    # ONNX Runtime execution providers in priority order.
+    # Auto-detected at startup; falls back to CPU if GPU is unavailable.
+    # Options: "CUDAExecutionProvider", "ROCMExecutionProvider",
+    #          "OpenVINOExecutionProvider", "CPUExecutionProvider"
+    execution_providers: list[str] = ["CPUExecutionProvider"]
     embedder: EmbedderSettings = EmbedderSettings()
     ocr: OCRSettings = OCRSettings()
     detector: DetectorSettings = DetectorSettings()
@@ -183,9 +188,22 @@ class Settings(BaseSettings):
     def performance(self) -> PerformanceSettings:
         return self.config.performance
 
+    # ONNX Runtime execution providers, overridable via env var.
+    # Comma-separated: "CUDAExecutionProvider,CPUExecutionProvider"
+    # or "auto" for GPU auto-detection.
+    vision_execution_providers: str = Field(
+        default="CPUExecutionProvider",
+        env="VISION_EXECUTION_PROVIDERS",
+    )
+
     @property
     def vision(self) -> VisionSettings:
-        return self.config.vision
+        v = self.config.vision
+        # Override execution_providers from env if set.
+        providers = [p.strip() for p in self.vision_execution_providers.split(",") if p.strip()]
+        if providers:
+            v.execution_providers = providers
+        return v
 
     class Config:
         env_file = ".env"