Function addOrMergeIngredientMatches

Adds or merges ingredients with existing ones.

Merging behavior:

• If ingredient doesn't exist: adds it (with its note if present)
• If ingredient exists with same unit: merges quantities, preserves new note
• If ingredient exists with different unit: replaces it entirely

Note handling: When merging quantities, the new ingredient's note takes precedence. This allows scraped data to provide usage context.