fix: retain link score fallbacks when preview filters match nothing

[linjiaye929]

Summary

When preview filters match no links, extract_link_heads() returns before applying the existing score fallback. Links with an intrinsic score therefore retain total_score=None; the same excluded link gets its fallback if another URL happens to match.

Route the empty selection through _merge_head_data(links, [], config). Filtering still prevents head fetching, and the existing scoring rules are unchanged. This covers the early-return path left outside #1761.

Verification

• New public-method regressions: 12 fail on unchanged develop, 5 controls pass; all 17 pass after the fix.
• 27 tests pass including existing merge/scoring regressions. Coverage includes include/exclude filters, external-only links, scoring on/off, missing/failed heads, weighted scores and empty links.
• Tests control only the initialized seeder fetch boundary; no live crawl, model calls or network attempts. New tests pass Black and diff checks; pre-existing production formatting is left unchanged. Full suite was not run.

Run pytest tests/unit/test_link_preview_filter_scoring.py tests/test_merge_head_data_scoring.py.

AI assistance was used for implementation and verification.

Hosted checks

At head 56c4ec61fd443247d14f1fdf703aec49e8a7cd4d, the Discord notification workflow is action_required; there are zero check runs. This does not establish hosted code/test CI passing.