Move MODS and TFE XML out of SQL storage

[candyhazlett]

Description

mods_xml (:mediumtext) and tfe_xml (:text) are currently stored as SQL columns on core_files (migration 20260113171831_add_tapas_xq_fields_to_core_files.rb). @amclark42 flagged two concerns: character encoding issues (XML encoding declarations can conflict with MySQL's own encoding settings), and queryability (XML stored as a text column isn't queryable in any meaningful way). TAPAS already uses Active Storage for the TEI file; applying the same pattern to MODS and TFE output would be more consistent and avoid these issues. Surfaced in PR #29 review.

Steps

1. Add has_one_attached :mods_file and has_one_attached :tfe_file (or equivalent) to CoreFile
2. Update ProcessTeiFileJob to write MODS/TFE content as file attachments rather than SQL columns
3. Update any other code that reads mods_xml or tfe_xml to use the new attachments
4. Write a migration to remove the mods_xml and tfe_xml columns
5. Update specs

Acceptance criteria

• mods_xml and tfe_xml columns removed from the core_files table
• MODS and TFE data stored via Active Storage attachments
• ProcessTeiFileJob successfully writes and reads MODS/TFE data through the new mechanism
• No character encoding issues
• Existing job specs pass

[candyhazlett]

RetrievalService dependency

Implementing this issue will require deciding how TapasXq::RetrievalService fits into the new storage model. Currently the service wraps all three GET endpoints but is only called from the tapas_xq:sync_mods rake task — retrieve_tei and retrieve_tfe are not called from anywhere in the app.

The relevant questions per method:

• retrieve_mods — currently the rake task re-fetches MODS from tapas-xq and overwrites mods_xml. Once MODS moves to ActiveStorage, does Rails re-fetch via retrieve_mods to write the attachment, or does it just store what ProcessTeiFileJob already received from the initial store call? The rake task's role may change or be removed depending on the answer.
• retrieve_tfe — not called from anywhere. Once TFE moves to ActiveStorage, is there a scenario where Rails needs to re-fetch TFE from tapas-xq (e.g., to re-sync visibility state), or does Rails always write TFE from its own data?
• retrieve_tei — not called from anywhere. The TEI file is already in ActiveStorage. Should Rails ever proxy the TEI back from tapas-xq, or is the ActiveStorage attachment always the source of truth?

Suggesting these decisions be made explicit as part of the acceptance criteria for this issue, so that RetrievalService ends up either wired into the new flow or documented as intentionally unused.