[EPIC] Inventory-beta.data.gov Redesign

### Summary

inventory.data.gov serves agencies that lack their own data cataloging software. It is currently built on CKAN. A full rebuild decision -- CKAN vs. a new approach -- is deferred to an October 2026 scoping sprint. This epic is the holding space for that work and the research that feeds the decision.

The long-term goal is to help agencies prepare and maintain DCAT-US 3.0 compliant metadata files, including sub-agencies building partial catalogs and data owners (database admins, project leads, etc.) editing metadata directly. Any solution must support team collaboration across time and preserve a persistent, single source of truth for agency metadata.

### Key considerations

- inventory.data.gov must remain a metadata tool only; it should not store or host datasets
- CKAN has never been made Section 508 compliant; this is a factor in the rebuild decision (note: CKAN may be moving toward compliance -- worth confirming before deciding)
- Nested or complex metadata (list-valued DCAT-US 3.0 properties) must be handled gracefully; JSON arrays are difficult to manage in flat interfaces
- Collaboration (multiple users, persistent state) is a requirement; client-side-only solutions do not meet this need
- GSA is currently the only agency hosting data files inside inventory.data.gov; a spike to understand implications for the redesign is recommended before the October decision, but resolving file hosting is not a prerequisite
- Updating a dataset's metadata should not interrupt access to the associated download file

### Short-term goal (see also #5974)

Allow agencies to export current inventory metadata as DCAT-US 3.0 compliant files using the existing 1.1 to 3.0 translation script. This is tracked under the short-term inventory upgrades epic.

- [ ] Import tool for small agency catalogs (to be scoped after October 2026 inventory decision --
previous ticket #5477 closed as not planned; concept is valid but needs a fresh approach
aligned to whatever inventory-beta becomes)
- [ ] Format conversion tool (to be scoped after October 2026 inventory decision --
previous ticket #5478 open but not planned; concept is valid but needs a fresh approach
aligned to whatever inventory-beta becomes)