Unify metadata attachments across web and desktop

[PaulHax]

Second of a four-PR stack, on top of #1806. Review only this PR's own commits; the base branch
carries #1806's changes.

Summary

One optional metadata attachment per dataset scope, consistent across web and desktop.

Single-camera datasets get one shared attachment; multicamera datasets get one shared parent
attachment plus one camera-local attachment per camera, camera-local taking precedence. Image
sequences and videos are supported from the start. JSON, TXT, and CSV stay opaque pipeline inputs
here — row parsing is the next PR.

Carried through: web and desktop creation-time selection; headless and RPC ingestion; single-camera
and multicamera imports; clone-root lookup; export/import round trips; pipeline discovery and opt-in
injection; and a normalized GET /dive_dataset/:id/frame_metadata_sources.

Reserved frame-metadata.* and frame_metadata.* names are recognized identically by Python and
TypeScript, pinned by a shared conformance fixture, and never sent through annotation parsing.

The read path — the source endpoint, the web and desktop loaders, and loadFrameMetadata on the
Api interface — lands here with no production consumer, because it is the attachment contract
and the next PR is its first caller. Review it against test_frame_metadata_sources.py, not a UI.

Two changes outside the attachment story ride along:

• One annotation CSV per sweep. process_items imported every CSV, last write winning. It now
  imports the oldest and names the rest in a warning, so the headless sweep enforces the rule Upload exactly the server-validated package #1806
  enforces interactively. The sweep is already rewritten here to pre-scan for reserved names.
• Metadata-item download path. _inject_dataset_metadata_file reconstructed <dir>/<item name>,
  wrong when girder_client sanitizes or nests the name — in the nested case exists() passes and a
  directory is bound into the KWIVER setting. It now locates what actually landed.

The write-only metadataFile item marker and its helper are dropped; nothing ever read it, and the
folder's metadataFileItemId is the only attachment locator. Items in existing datasets keep the
stale key harmlessly.

Reviewer focus

• Attachment identity is stored once per scope and stays inside the owning dataset/clone root.
• Camera-local attachments override shared ones without hiding shared data from other cameras.
• Attachment bytes stay opaque to the server.
• Pipeline injection happens only for pipelines declaring a metadata-file key.
• Clone/export/import preserve original names and create no annotation tracks.
• Replacing an attachment deletes the superseded file only when DIVE stored it. A reserved-name file
  is the user's own content, uploaded with the media and merely discovered — and process_items
  records what it discovers in the same marker, so ownership is decided by the reserved-name
  predicate, not the marker's presence.
• A camera's attachment is the optional third argument of Upload exactly the server-validated package #1806's camera package. Unlike the
  annotation pick it is removed from the package rather than swapped into it — it is uploaded and
  declared separately once the camera dataset exists — and a folder file it displaces is still
  reported.
• Metadata selections use opaque per-selection ids, so same-named attachments picked from different
  directories for different cameras do not collide.

Manual test

Uses okeanos-frame-metadata-upload, okeanos-video-explicit-frame, and okeanos-video-opaque-json
from okeanos-frame-metadata-fixtures.zip, attached to #1806 — CC0 NOAA Okeanos Explorer media with
synthetic metadata and annotations. Select all files in one directory per test.

1. Select all files; confirm the reserved name takes the Metadata File slot, not the annotation slot.
   In okeanos-frame-metadata-upload both slots fill at once: annotations.csv is the
   annotation pick and frame-metadata.csv the attachment, from a single selection of two CSVs.
2. Create and open: expected media, and the only tracks are the two from the VIAME CSV — the
   attachment contributes none.
3. GET /api/v1/dive_dataset/<folder-id>/frame_metadata_sources — one shared object with an
   itemId, the original name, and an empty cameras object.
4. Clone the image dataset; the same request against the clone still resolves through the clone root.
5. For the JSON case, the endpoint reports frame_metadata.json — stored for pipelines, not
   interpreted, and the underscore spelling is discovered exactly like the hyphen one.

Desktop smoke test: import a media directory, choose its sidecar in Metadata File,
export the archive, reopen, and confirm the attachment name survives.