Refuse a frame the renderer cannot draw, and honour two config keys

[nmbrthirteen]

A thumbnail is an HTML page photographed by headless Chrome, so the frame is an <img>. A file the browser cannot decode was never an error: the page rendered anyway, the caption box and the logo landed on bg_color, and thumbnail-render exited 0 with a path to a blank card carrying a 15px broken-image glyph in the corner. On a dark template that is a black rectangle nobody can tell apart from a picture that worked, and every caller downstream stored it as a finished thumbnail.

A missing frame was worse, because it also moved the text box to the no-photo height, so the failure was visible only as a picture that sat slightly wrong.

Both are refused now, in the words of what happened, while the answer is still a message about one picture rather than a card somebody ships.

Two other things in the same area:

• The file:// URIs for the photo and the logo were built by string concatenation, so a # or a ? anywhere in the path was a fragment or a query to the browser and the image silently dropped, rendering as bg_color the same way. They use Path.as_uri() now.
• photo_object_position was asked of the model, exposed in the template editor and then never read: the CSS hardcoded center center. line1_nowrap shipped in the defaults and was documented as tunable while the renderer overwrote it unconditionally. Both now do what they say.

Verification

tests/test_thumbnail_html.py passes (11), and the CLI was run against a probe matrix:

frame	before	after
valid PNG	exit 0, photo drawn	exit 0, photo drawn
TIFF	exit 0, blank card	exit 1, names the format
S3 XML error body named .png	exit 0, blank card	exit 1, "is not an image"
0-byte file	exit 0, blank card	exit 1, "is not an image"
missing path	exit 0, blank card at wrong height	exit 1, "no frame at ..."

A render with photo_object_position and line1_nowrap set was confirmed to pick both up.

Context

Found while tracing a podcli-cloud report: a workspace picked one of its own background images for a clip thumbnail and got back three pure black cards. The cloud-side half is fixed separately; this is the engine half, and it is what turns that class of failure from a silent blank card into a job that says why.

[coderabbitai]

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Reviewing files that changed from the base of the PR and between 7f10c42 and 07ff31e.

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• backend/cli.py
• backend/services/thumbnail_html.py

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