feat: PostgreSQL-backed dataset search with filters, sync pipeline, and file download

[elainefan331]

Description:
Overview
Replaces the legacy CGI-based search with a full PostgreSQL implementation, introduces a rich filter UI, and adds file download capabilities for search results.

Backend & Infrastructure

• Added search tables migration and updated DB config for local Postgres
• Replaced CGI search with direct PostgreSQL queries in searchAllDatabases
• Added incremental sync script to populate ioviews and iolinks from CouchDB, refactored with transactions, concurrency, and error handling
• Sync now pulls database list from registry

Search Filters

• Modality-aware combobox for Data type filter
• Draggable age-range slider with per-handle clearing
• Multi-select File types filter (dataset-level) backed by iolinks table
• Dataset-level modality filter with AND/OR mode
• Placeholder hints on task/session/run keyword fields
• Min/max count fields paired on one row
• Run count shown in subject card

Resolves #82

• Added incremental sync script to populate ioviews and iolinks from CouchDB
• Sync now pulls database list from registry

Resolves #83

• Replaced CGI search with direct PostgreSQL queries in searchAllDatabases
• Added file type filter to search using iolinks table
• Added file download endpoints for dataset search results
• Mac/Linux and Windows script options for downloading filtered files