feat(scan): per-field artwork priority overrides

[gantoine]

Description
^{Explain the changes or enhancements you are proposing with this pull request.}

Closes #3812.

Adds optional per-field artwork priority so users can prioritize metadata sources differently for each artwork field, instead of one shared scan.priority.artwork list applying to covers, screenshots, and manuals alike.

New optional config.yml keys, each falling back to scan.priority.artwork when omitted:

scan:
  priority:
    artwork:        # shared fallback (unchanged)
      - igdb
      - ss
    cover:          # cover art only
      - ss
      - tgdb
    screenshot:     # screenshots only
      - igdb
    manual:         # game manuals only
      - launchbox

This resolves the issue's core case: e.g. keep original box art for covers while sourcing screenshots from a different provider.

Scope / notes

• Fully backward compatible: absent keys leave scanning behavior unchanged.
• Config-only for now (priority is hand-edited YAML, as today); exposed read-only via GET /api/config.
• The SGDB cover tie-break now respects the cover-specific order.
• Intentionally out of scope (can follow up): new titles/video media types (would need new Rom columns + migration) and region auto-matching. See discussion on the issue.

AI assistance disclosure
^{Per CONTRIBUTING.md.}

This PR was implemented with AI assistance (Claude Code). The design, code, tests, and docs were AI-generated under human direction and reviewed before submission.

Checklist
^{Please check all that apply.}

• I've tested the changes locally
• I've updated relevant comments
• I've assigned reviewers for this PR
• I've added unit tests that cover the changes

Screenshots (if applicable)

N/A (backend + config, no UI changes).

🤖 Generated with Claude Code

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[greptile-apps]

Greptile Summary

This PR adds independent metadata-source priorities for artwork fields. The main changes are:

• Optional cover, screenshot, and manual priority lists with shared artwork fallback.
• Per-field source ordering during ROM scans.
• Cover-specific SGDB tie-breaking.
• Backend API and frontend model support for the override map.
• Configuration examples and focused priority tests.

Confidence Score: 4/5

Per-field overrides can crash scans when they contain an unknown provider name.

• The new lists are validated only as lists, not as known metadata sources.
• An invalid entry reaches an unguarded enum conversion during scanning.
• Omitted overrides, API serialization, and valid per-field ordering otherwise remain consistent.

backend/config/config_manager.py and backend/handler/scan_handler.py

Important Files Changed

Filename	Overview
backend/config/config_manager.py	Parses, validates, and exports the new per-field artwork priority lists, but validation does not reject unknown provider names.
backend/handler/scan_handler.py	Applies field-specific artwork ordering and SGDB cover ranking; an unknown provider in a new override can raise during a scan.
backend/endpoints/configs.py	Exposes the artwork override map through the configuration endpoint.
backend/endpoints/responses/config.py	Adds the artwork override map to the backend response type.
backend/tests/handler/test_scan_priority.py	Covers fallback and ordering behavior but does not cover invalid provider names in per-field overrides.
frontend/src/generated/models/ConfigResponse.ts	Adds the new override map to the generated frontend response model.
frontend/src/stores/config.ts	Adds an empty artwork override map to the frontend configuration defaults.

Prompt To Fix All With AI

Fix the following 1 code review issue. Work through them one at a time, proposing concise fixes.

---

### Issue 1 of 1
backend/handler/scan_handler.py:124-126
**Unknown Override Source Crashes Scan**

When a new per-field list contains an unknown provider name, such as a typo in `scan.priority.cover`, this override reaches `MetadataSource(source)` without element validation and raises `ValueError` during scanning. Config validation currently accepts any list, so the application starts successfully but the affected scan fails instead of rejecting the invalid setting.

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