feat: add vision pipeline scaffolding with ONNX backend

Introduce the app/services/vision/ module with ABC interfaces, ONNX
Runtime backend, model registry, and per-task implementations:
- OpenCLIP ViT-B/32 embedder (image + text, 512-d)
- RapidOCR engine (PP-OCRv4 via ONNX, no PaddlePaddle)
- YOLOv8n object detector (raw ONNX, no ultralytics runtime)
- YuNet + SFace face processor (Apache 2.0, opencv_zoo, 128-d)
- DBSCAN face clustering helper

Add VisionSettings to config (mulita.yml + Pydantic), bootstrap_models.py
for first-boot weight downloads, models_data Docker volume, and ROCm
backend stub for future GPU acceleration.

No Celery tasks wired yet — models load but nothing invokes them.

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

backend/app/config.py

@@ -29,6 +29,42 @@ class PerformanceSettings(BaseModel):
     db_pool_size: int = 20
     db_pool_recycle: int = 3600
 
+class EmbedderSettings(BaseModel):
+    """CLIP / SigLIP embedding model settings"""
+    name: str = "openclip_vitb32"
+    batch_size: int = 8
+
+class OCRSettings(BaseModel):
+    """PaddleOCR / rapidocr settings"""
+    enabled: bool = True
+    languages: list[str] = ["en"]
+    min_confidence: float = 0.5
+
+class DetectorSettings(BaseModel):
+    """YOLOv8n object detection settings"""
+    enabled: bool = True
+    min_confidence: float = 0.35
+    max_detections: int = 50
+
+class FacesSettings(BaseModel):
+    """YuNet + SFace face detection/recognition settings"""
+    enabled: bool = True
+    min_face_size: int = 40
+    recognition_threshold: float = 0.4
+    cluster_eps: float = 0.35
+
+class VisionSettings(BaseModel):
+    """AI vision pipeline settings. Disabled when running on SQLite
+    (pgvector is required for embedding storage)."""
+    enabled: bool = True
+    backend: str = "onnx"           # "onnx" | "rocm" (future)
+    models_dir: str = "/data/models"
+    embedder: EmbedderSettings = EmbedderSettings()
+    ocr: OCRSettings = OCRSettings()
+    detector: DetectorSettings = DetectorSettings()
+    faces: FacesSettings = FacesSettings()
+    worker_concurrency: int = 2
+
 class MulitaConfig(BaseModel):
     """Main configuration from YAML file. Source roots and the discard
     workflow are owned by the database now — only operational settings
@@ -36,6 +72,7 @@ class MulitaConfig(BaseModel):
     thumbnails: ThumbnailSettings = ThumbnailSettings()
     scanner: ScannerSettings = ScannerSettings()
     performance: PerformanceSettings = PerformanceSettings()
+    vision: VisionSettings = VisionSettings()
 
 class Settings(BaseSettings):
     """Application settings"""
@@ -127,6 +164,10 @@ class Settings(BaseSettings):
     def performance(self) -> PerformanceSettings:
         return self.config.performance
 
+    @property
+    def vision(self) -> VisionSettings:
+        return self.config.vision
+
     class Config:
         env_file = ".env"
         case_sensitive = False