feat: batch image deletion with a single embeddings-index rewrite

[lstein]

Summary

Deleting N starred images from the favorites menu previously issued N serial DELETE /delete_image requests, and each one loaded and atomically rewrote the entire .npz index — O(N × index size) I/O that made multi-select deletes crawl on large albums. Deleting 20 favorites from a 50k-image album meant reading and rewriting the full index 20 times.

This adds a batch path that does the whole selection with one request and one index rewrite, so a multi-select delete now costs the same index I/O as deleting a single image.

Changes

Backend

• New POST /delete_images/{album_key} endpoint takes {indices, move_to_trash}, resolves every index against one snapshot of the sort order, deletes the files, and drops all their rows in a single rewrite. Per-item failures (out-of-range index, missing file, access denied) are reported in an errors list without failing the batch.
• For InvokeAI board albums, each file deletion still routes through the InvokeAI API (it owns the files and its DB rows), but the index rewrite is batched into one at the end. A per-image InvokeAI failure keeps that row in the index and deletes the rest.
• New Embeddings.remove_images_from_embeddings(indices) maps all sorted-order indices through the (modtime, filename) lexsort against one snapshot — callers need no reverse-order shifting — and its single atomic_savez carries over the non-per-image keys (model_id etc., per fix: deleting an image no longer breaks search or drops out of search mode #350) so the encoder stamp survives. The singular remove_image_from_embeddings now delegates to it.

Frontend

• New deleteImages() helper in index.js.
• deleteBookmarkedImages() in bookmarks.js sends one batch request instead of a serial per-image loop, and dispatches the existing albumChanged deletion event with the indices the server actually removed.

Testing

• New backend tests: batch delete happy path (rows dropped, files gone, encoder stamp preserved), per-item error reporting for bad indices, empty-list rejection, sorted-index→raw-index mapping with a deliberately shuffled sort order, and two InvokeAI-board tests (routing through the API, partial-failure keeps the failed row).
• New Jest tests for deleteImages() request shape and error propagation.
• Full suites green: 41 backend (test_index + test_invokeai_board_index), 424 frontend; ruff / eslint / prettier clean.
• Manually verified in the running app: multi-select favorites delete completes in roughly single-delete time.

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