Restore recipe search paging

[MikeChristensen]

Summary

• return the total number of matching recipes independently of the current 100-result page
• apply the same paging contract to database-backed and in-memory search providers
• add a stable RecipeId tie-breaker so adjacent offset pages cannot reorder equal-valued results
• expose the shared 100-result page size through RecipeQuery.PageSize
• add a fast regression test covering full, partial, and beyond-end pages

Validation

• dotnet build DB/DB.csproj --configuration Release --no-restore
• dotnet test UnitTests/UnitTests.csproj --configuration Release --no-restore (22 passed)
• exercised the database provider through the Website API against PostgreSQL
• verified 55,102 total matches with 100 results at offsets 0 and 100
• verified no duplicate IDs and stable ordering between adjacent pages
• verified an offset beyond the result set returns an empty page while preserving the total count
• verified filtered adjacent pages report the same total count

A matching Core/DB v1.0.1 release can be tagged after this PR is merged.