Add metadata and capture roll/frame to export filename templates, with safe frame padding and pre-export settings flush

[jboneng]

Summary

Export filename templates can now include analog metadata and Scanlight capture identity (roll, frame, camera, film, etc.). Capture roll/frame are stamped automatically on import and editable in the Metadata panel. A pre-export flush ensures the export always uses the latest sidebar settings, even when the user clicks Export before the 500ms debounce has fired.

New filename template variables

All metadata variables are read from the file’s MetadataConfig at export time (the Metadata panel for the current frame, or each file’s saved config in a batch). Path-unsafe characters are stripped; empty fields render as empty strings.

Variable	Source	Notes
{{ roll }}	MetadataConfig.capture_roll	Scanlight capture roll name. Stamped on capture import; editable in Metadata → Scanning → Roll / Frame. Fallback: parsed from a {roll}_Frame{NNN} source filename stem. Not the Roll Analysis normalization name (process.roll_name).
{{ frame }}	MetadataConfig.capture_frame	Capture frame number (integer). Stamped on capture import; editable in Metadata → Roll / Frame. Fallback: parsed from the same {roll}_Frame{NNN} stem. Unset (none) if unknown.
{{ frame_padded }}	Derived from frame	Zero-padded frame (012). Empty when frame is unset. Same as {{ frame|pad(3) }}.
{{ frame|pad(N) }}	Derived from frame	Jinja pad filter — zero-pads to N digits; empty when frame is unset (avoids "%03d" % frame crashing the whole template).
{{ camera }}	camera_make + camera_model	Combined as "Make Model", trimmed.
{{ camera_make }}	MetadataConfig.camera_make	From gear library or manual Metadata entry.
{{ camera_model }}	MetadataConfig.camera_model	From gear library or manual Metadata entry.
{{ lens }}	lens_model or lens_make	Model if set, otherwise make.
{{ lens_make }}	MetadataConfig.lens_make	From gear library or manual entry.
{{ lens_model }}	MetadataConfig.lens_model	From gear library or manual entry.
{{ focal_length }}	MetadataConfig.focal_length_mm	Lens focal length in mm.
{{ film }}	MetadataConfig.film	Film stock name.
{{ film_iso }}	MetadataConfig.film_iso	Film ISO.
{{ film_manufacturer }}	MetadataConfig.film_manufacturer	Film manufacturer.
{{ film_color_type }}	MetadataConfig.film_color_type	e.g. Color negative.
{{ film_format }}	MetadataConfig.format / format_other	Film format (35mm, 120, …). Uses format_other when format is "Other". Distinct from export {{ format }} (JPEG/TIFF).
{{ developer }}	MetadataConfig.developer	Developer.
{{ push_pull }}	MetadataConfig.push_pull	Integer −3…+3 (0 = Normal).
{{ scanning }}	MetadataConfig.scanning	Scanning method note.
{{ exposure }}	MetadataConfig.exposure_override	Free-text exposure note.

Existing export variables (original_name, colorspace, format, paper_ratio, size, dpi, target_px, border, date) are unchanged.

Example

{% if roll %}{{ roll }}{% endif %}{% if frame %}Frame{{ frame|pad(3) }}{% endif %}{{ film }}{{ film_iso }}{{ camera }}_{{ original_name }}

Use {% if %} blocks when a variable may be empty — otherwise literal labels like Frame can remain in the filename even when roll/frame are unset.

Pre-export settings flush

The Export sidebar debounces changes to Format / Size / Colour / Destination for 500ms before writing them into WorkspaceConfig. Without a flush, clicking Export (or Export Presets) immediately after editing the filename pattern or destination could run against stale config.

This PR adds _flush_export_settings() on the export sidebar, registered on the controller as flush_export_settings. Before any export path runs, the controller calls _flush_export_ui(), which stops the debounce timer and persists the form immediately. Covered entry points:

• Export current frame / selected / batch (chevron menu)
• Export Presets
• Contact sheet

Other changes

• Metadata panel: Roll / Frame fields under Scanning (backed by capture_roll / capture_frame).
• Capture import: Scanlight roll_name and frame_number are stamped into metadata when a captured file is opened.
• Template failures: Jinja render errors are logged before falling back to original_name.
• Docs: docs/TEMPLATING.md and docs/USER_GUIDE.md updated.

Test plan

• Fill Metadata (camera, film, ISO) and export with a template containing those variables — filename reflects the values.
• Capture a frame via Scanlight — roll/frame appear in Metadata and in export filenames.
• Export a non-capture file with {{ frame|pad(3) }} in the pattern — template does not fall back to original_name.
• Change the filename pattern or destination, click Export within 500ms — export uses the new settings.
• Export Presets with a metadata template — each preset uses its own filename pattern and destination.
  Fixes expand export filename templates with metadata variables #704