feat: guard the paths that assume an album entry is a still image

[lstein]

PR 6 of 8 in the video-support stack. Stacked on #359.

Six places would misbehave once the directory walk starts collecting videos. Each is fixed and pinned here, ahead of the flip in PR 7.

find_duplicate_clusters excludes videos

A video's embedding describes one extracted frame, and opening frames are so often a black slate or title card that two unrelated clips sit at ~1.0 cosine and get reported as duplicates — which users act on by deleting. Videos were already excluded from dedupe as a product decision; nothing enforced it.

_load_image_path raises 400 for videos

It's the choke point for /invokeai/use_ref_image, which POSTs the file to InvokeAI's /images/upload. An .mp4 would come back as an opaque 502 carrying raw upstream text. The drawer already withholds the button for videos (PR 3); this closes the API path behind it.

Worth noting for review: /recall goes through _load_raw_metadata, not the file path, so it already refused videos for a different reason. There's a test pinning that too, so a future refactor routing recall through the file path stays correct.

search_with_text_and_image returns 400, not 500

A video dropped on the search panel used to surface as an opaque error from deep inside PIL.

download_images_zip

Video members are ZIP_STORED rather than deflated (their streams are already compressed), and selections over 2 GB get a 413. The archive is assembled entirely in BytesIO, so twenty bookmarked 200 MB clips would have been several gigabytes resident — an OOM only reachable once videos exist.

index_metadata splits the count

Returns image_count and video_count alongside filename_count, so the album card can explain why the number jumped. Legacy indexes report video_count == 0.

progress_tracker.add_completion_warning

The completion warning was a single slot, already used for the board-album "N of M missing" notice — a video-skip warning would have silently discarded it. A run can legitimately produce both. set_completion_warning keeps its replace semantics.

bookmarks.js video download

Points an <a download> straight at the URL instead of buffering the whole file into a blob (a 200 MB clip would sit entirely in browser memory before the save dialog appeared), and derives the fallback extension from the real path rather than hardcoding .jpg.

Tests: 14 new in test_video_guards.py. Backend 590 passed, frontend 501 passed, ruff + eslint clean.

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