Plugin: Unified metadata storage and semantic search service

Multiple plugins extract different types of metadata from evidence (face attributes from age-gender detection, scene descriptions from image captioning, face recognition etc.). Currently, there's no unified way to store, search, or filter files based on this accumulated metadata.

Create a metadata storage and search service that allows plugins to write structured attributes and descriptions, and enables users to search/filter files based on this metadata. The service should support semantic search on text descriptions using embeddings, exact/approximate text matching on attributes, and combined multi-attribute filtering. This enables powerful cross-plugin queries like "find all images of males aged 25-32 in mountain resort like areas" by combining results from multiple analysis plugins.

This issue spans across multiple plugins and involves refactoring multiple plugins while also creating a unified metadata store. All results from plugins must be written to this store to enable search. Relates to multiple existing issues linked below.