feat: asset-based image/video sources, notification sounds, UI improvements

- Replace URL-based image_source/url fields with image_asset_id/video_asset_id
  on StaticImagePictureSource and VideoCaptureSource (clean break, no migration)
- Resolve asset IDs to file paths at runtime via AssetStore.get_file_path()
- Add EntitySelect asset pickers for image/video in stream editor modal
- Add notification sound configuration (global sound + per-app overrides)
- Unify per-app color and sound overrides into single "Per-App Overrides" section
- Persist notification history between server restarts
- Add asset management system (upload, edit, delete, soft-delete)
- Replace emoji buttons with SVG icons throughout UI
- Various backend improvements: SQLite stores, auth, backup, MQTT, webhooks

server/src/wled_controller/storage/asset.py

@@ -0,0 +1,80 @@
+"""Asset data model.
+
+An Asset represents an uploaded file (sound, image, video, or other)
+stored on the server. Assets are referenced by ID from other entities
+(e.g. NotificationColorStripSource uses sound assets for alert sounds).
+"""
+
+from dataclasses import dataclass, field
+from datetime import datetime
+from typing import List, Optional
+
+
+# Map MIME type prefixes to asset_type categories
+_MIME_TO_ASSET_TYPE = {
+    "audio/": "sound",
+    "image/": "image",
+    "video/": "video",
+}
+
+
+def asset_type_from_mime(mime_type: str) -> str:
+    """Derive asset_type from a MIME type string."""
+    for prefix, asset_type in _MIME_TO_ASSET_TYPE.items():
+        if mime_type.startswith(prefix):
+            return asset_type
+    return "other"
+
+
+@dataclass
+class Asset:
+    """Persistent metadata for an uploaded file asset."""
+
+    id: str
+    name: str
+    filename: str                    # original upload filename
+    stored_filename: str             # on-disk filename (uuid-based)
+    mime_type: str                   # e.g. "audio/wav", "image/png"
+    asset_type: str                  # "sound" | "image" | "video" | "other"
+    size_bytes: int
+    created_at: datetime
+    updated_at: datetime
+    description: Optional[str] = None
+    tags: List[str] = field(default_factory=list)
+    prebuilt: bool = False           # True for shipped assets
+    deleted: bool = False            # soft-delete for prebuilt assets
+
+    def to_dict(self) -> dict:
+        return {
+            "id": self.id,
+            "name": self.name,
+            "filename": self.filename,
+            "stored_filename": self.stored_filename,
+            "mime_type": self.mime_type,
+            "asset_type": self.asset_type,
+            "size_bytes": self.size_bytes,
+            "description": self.description,
+            "tags": self.tags,
+            "prebuilt": self.prebuilt,
+            "deleted": self.deleted,
+            "created_at": self.created_at.isoformat(),
+            "updated_at": self.updated_at.isoformat(),
+        }
+
+    @staticmethod
+    def from_dict(data: dict) -> "Asset":
+        return Asset(
+            id=data["id"],
+            name=data["name"],
+            filename=data.get("filename", ""),
+            stored_filename=data.get("stored_filename", ""),
+            mime_type=data.get("mime_type", "application/octet-stream"),
+            asset_type=data.get("asset_type", "other"),
+            size_bytes=int(data.get("size_bytes", 0)),
+            description=data.get("description"),
+            tags=data.get("tags", []),
+            prebuilt=bool(data.get("prebuilt", False)),
+            deleted=bool(data.get("deleted", False)),
+            created_at=datetime.fromisoformat(data["created_at"]),
+            updated_at=datetime.fromisoformat(data["updated_at"]),
+        )